CIHM 
Microfiche 
Series 
(Monographs) 


ICMH 

Collection  de 
microfiches 
(monographies) 


Canadian  Instltuta  for  Historical  IMIcioraproductians  /  Inititut  Canadian  da  i 


I  liistoriquaa 


1995 


Technical  and  Bibliographic  Notes  /  Notes  technique  et  biWiographiques 


The  Institute  has  attempted  to  obtain  the  best  original 
copy  available  lor  filming.  Features  of  this  copy  which 
may  be  bibliographically  unique,  which  may  alter  any  of 
the  images  in  the  reproduction,  or  which  may 
significantly  change  the  usual  method  of  filming  are 
checked  below. 


D 


D 
D 


D 


D 


CokHJtBd  coven  / 
Couverturs  de  couleur 


I     I  Covers  damaged  / 

' — '  Couverture  endommagie 

I     I  Covers  restored  an*or  laminated/ 

' — '  Couverture  lestauree  efou  pellicuiee 

I     I  Cover  title  missing /Letitredecouveilure  manque 

I     I  Cotoured  maps/ Cartes  gSogiaphiquos  en  couleur 

r~]  Coloured  ink  (l.e.  other  than  blue  or  black)  / 

—  Encra  de  couleur  (i.e.  autre  que  Weueou  noire) 

r/T  Coloured  plates  and/or  illustraltons/ 

' — '  PlanctMset/oulllustratnns  en  couleur 

I     I  Bound  >vlth  other  material/ 

— '  Rell*  avec  d'autres  documents 


Only  editk>n  available  / 
Seule  Mltk>n  disponible 

Tight  binding  may  cause  shadows  or  distortion 
along  Interior  margin  /  La  reliure  serree  peut 
causer  de  I'omtHe  ou  de  la  distorston  le  long  de 
la  marge  inteiieure. 

Blank  leaves  added  during  rastoratnns  may  appear 
within  the  text.  Whenever  possible,  these  have 
been  omitted  fram  timing  /  II  se  peut  que  ceririnas 
pages  blanches  ajoutees  kits  dune  restauratkxi 
appaialssant  dans  le  te:.:<>,  mais,  knque  cela  «tait 
poaaiUe,  oes  pages  n'cnt  pas  eie  tfentes. 


Addtional  comments  / 
Commentaires  suppMmentairas: 


Thh  itsin  it  lilmad  at  ttw  rtduelion  ratio  diseksd  below/ 

Cc  doctmnnt  tn  f  ilm4  »i  tavi  <te  rMvetion  inH^ut  c<-dttMu>. 


L'Institut  a  microfilm*  le  meilleur  examplaire  qu'il  lui  a 
Hi  possible  de  se  procurer.  Les  d6tails  de  cet  exem- 
plalre  qui  sont  peut-dtre  uniques  du  point  de  vue  bibli- 
ographique,  qui  peuvent  modifier  une  image  reproduite, 
ou  qui  peuvent  exiger  une  modifications  dans  la  m6th- 
ode  nomiale  de  filmage  sont  indk|u6s  ci-dessous. 

I     [     Coloured  pages/ Pages  de  couleur 

rn     Pages  damaged/ Pages  endoimnagies 

I     I     Pages  restored  and/or  laminated/ 
— '     Pages  lestaurtes  et/ou  petlkajUes 


S" 


Pages  discokxired,  stained  or  foxed  / 
Pages  decok»«es,  tachettes  ou  pquees 


I     I     Pages  detached/ Pages  d«tach«es 

ra     Showlhrough  /  Transparence 

I     I     Quality  d  print  varies  / 

' — I     Quality  inigale  de  I'impiiBsston 

I     I     Includes  supplementary  material/ 
— '     Comprend  du  materiel  supplimentaire 

I  I  Pages  wholly  or  partially  obscured  by  errata 
slips,  tissues,  etc.,  have  been  refilmed  to 
ensure  the  best  possible  image  /  Les  pages 
totalement  ou  pertiellement  otncurcies  par  un 
feuillet  d'enata,  une  pelure,  etc.,  ont  et6  flimies 
li  nouveau  de  fa9on  6  obtenir  la  mellleure 
image  possible. 

I  I  Opposing  pages  with  varying  colouration  or 
discolourations  are  filmed  twkse  to  ensure  the 
best  possible  image  /  Les  pages  s'opposant 
ayant  des  cotorations  variables  ou  des  decol- 
oratk)ns  sont  lllmees  deux  fois  afin  d'obtenir  la 
meilleur  image  possible. 


lUX 

^,^^ 

^^^ 

^^^^ 

14X 

18X 

ax 

2tX 

»x 

J 

" 

12X 

1«X 

20X 

34X 

ax 

32* 

Th*  copy  filmed  har*  hM  baan  raproducad  thanki 
to  iha  ganareaitv  of: 

National  Library  of  Canada 


L'axamplaira  filmt  lut  raproduit  grica  t  la 
gtatroait*  da: 

Blbllothaqua  natlonala  du  Canada 


Tha  imagaa  appaaring  hafa  ara  tha  bast  quality 
poMlbla  eonaidaring  Uia  eendition  and  lagibillty 
of  tha  original  copy  and  in  kaoping  witfi  tha 
filming  contract  apacifleationa. 


La*  imagaa  tuivantaa  ont  tti  raproduitat  avac  la 
plui  grand  (oin,  compta  tanu  da  la  condition  at 
da  la  nattat*  da  I'aiamplaira  film*,  at  an 
eonformit*  avac  laa  condltiona  du  central  da 
filmaga. 


Original  eopioi  in  printed  paper  covers  ere  filmed 
beginning  with  the  front  cover  and  ending  on 
the  laat  page  with  a  printed  or  illustrated  impree- 
sion.  or  the  beck  cover  when  appropriate.  All 
other  original  copiae  ara  filmed  beginning  on  the 
first  pege  with  a  printed  or  illusuatad  Impree- 
sion,  end  ending  en  the  last  page  writh  e  printed 
or  illuatrated  impression. 


Laa  eaempleirea  originaux  dont  la  couvanure  an 
papier  eet  imprimte  aont  filmA*  en  commencant 
par  le  premier  plot  et  en  terminant  soit  par  la 
dernitre  page  qui  comporte  une  empreinte 
d'impression  ou  d'illustration,  soit  par  Is  sscond 
plat,  salon  la  cas.  Tous  las  autras  examplaira* 
originaus  sent  filmOs  sn  commencant  par  la 
premiere  pege  qui  comporte  une  empreinte 
dimpression  ou  d'lUustrstion  et  en  terminant  par 
la  damiAre  page  qui  comporte  une  telle 
empreinte. 


The  leat  recorded  freme  on  each  microfiche 
shall  contain  tha  symbol  -•■  Imeening  "CON- 
TINUED"), or  the  symbol  V  (meaning  "END"), 
whichever  appliaa. 

Mapa.  plates,  chsru.  etc..  may  be  filmed  at 
different  reduction  ratios.  Those  toe  large  to  be 
entirely  included  in  one  exposure  sre  filmed 
beginning  in  the  upper  left  hand  comer,  left  to 
right  end  top  to  boRom,  as  many  frames  aa 
required.  Tha  following  diagrams  illustrate  the 
method: 


Un  dee  symbolea  sulvanu  tpparaitra  sur  la 
dernMre  image  da  cheque  microfiche,  seion  le 
cas:  la  symbola  -»  signifie  "A  SUIVRE '.  le 
symbole  ▼  signlfie  "FIN". 

Les  cartas,  planchea.  ubiaaux.  etc..  peuvent  itra 
filmta  i  dee  Uux  da  rMuction  differenis. 
Lorsque  le  document  est  trop  grand  pour  itra 
raproduit  en  un  seul  clich*.  il  est  film*  A  pertir 
de  Tangle  supArieur  gauche,  de  gauche  t  droite. 
et  de  haut  en  bas,  en  prenant  le  nombre 
d'imegee  nAcessaire.  Les  diagrammes  suivants 
iUustrant  la  mtthode. 


1  2  3 


1 

2 

3 

4 

5 

6 

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(ANSI  ond  ISO  TEST  CHART  He.  2) 


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THE 
DEVELOPMENT  OF  THE  HUMAN  BOD\ 


McMURRICH 


MORRIS'S  ANATOMY 

POVRTH  EDITION 

UNDER  AMERICAN  EDITORSHIP 
*«*-..  lUvU^.  I^frav^^  ^,  y^  ^^  ^^^^^ 

ZOITBD  BY 

c  ^      HENRY  MORRIS.  F.R.C  S 

UnhenUy  «f  Dyrkam,  «c. 

AND 

-ii'.I^^ir^'fJI'^'^^'^'^'CH.  A.M..PH.D. 

B^rdeen  and  Florence  R  SaWn     H™^M      •'"^^-  •^""-  ^""rf"  «• 

of  ArrTJ^7i2S;^  to'r  "hS  *«*"  "-^  ""-"O  i.  «  ind« 
MtdkclRtard,  New  York        '^  ^  "commended  to  ,U  mtensted/'-FiS,  rS 

5'SI5S?°'»""!^-'!^'''is^•wi;^u.e-s      =■  ^  - 

Of  the  book  has  been  very  materially  inSSirtI  ""'  ""  "**•«  "<"• 

datJ'in"?^^ '"S^  ■" 'Pedal  «Jvantage  to  stude^^     I,  i.  m„H 

Sheep  or  Half  Moroo^,  iy^T^     o,1  j!"''^-    °''"''  »«•«»• 
each  part  sold  separately  "  "  ^"'^  '^^'^'  «»  fo"°»«. 

PART      '■~?*°'P;"«™""-    O^'^'ogy.    Articulation.     Index.     |,  „ 


THE 


DEVELOPMENT  OF  THE 
HUMAN  BODY 

A  MANUAL  OF  HUMAN  EMBRYOLOGY 


J.  PLAYFAIR  McMURRICH,  A.  M.,  Ph.  D.,  LL.  D. 

p>ons>o>  or  ahatovt  ik  nt  gmviRtrrr  o>  touohto 
rounuT  noruioi  or  akatoht  ih  thi  univimitt  or  mcwoAH 


FOURTH  EDITION,  REVISED  AND  tNLARGED 


With  Two  Hundred  and  Eighty-five  Illustrations  Several 
of  which  are  Printed  in  Colors 


PHILADELPHIA 
P.  BLAKISTON'S  SON  &  CO. 

1012  WALNUT  STREET 
1913 


i»'  i^if-iit 


r\\3 


CoFviKar,  ,,X3,  „  p.  Bux.noN-»  Son 


ft  Co. 


994tJ383 


TRB'MArL: 


a-PBKSO.TORK. 


PREFACE  TO  THE  FOURTH  EDITION. 


The  mcreasing  interest  in  human  and  mammalian  embryology 
which  has  characterized  the  last  few  years  has  resulted  in  many 
additions  to  our  knowledge  of  these  branches  of  science,  and  has 
necessitated  not  a  few  corrections  of  ideas  formerly  held  In  this 
fourth  edition  of  thU  book  the  attempt  has  been  made  to  incorpo- 
rate the  results  of  aU  important  recent  contributions  upon  the  topics 
discussed,  and,  at  the  same  time,  to  avoid  any  considerable  increase 
in  the  bulk  of  the  volume.  Several  chapters  have,  therefore,  been 
almost  entirely  recast,  and  the  subject  matter  has  been  thoroughly 
revised  throughout,  so  that  it  is  hoped  that  the  book  forms  an 
accurate  statement  of  our  present  knowledge  of  the  development 
of  the  human  body. 

To  several  coUeagues  the  author  is  indebted  for  valuable  sug- 
gestions, and  in  this  connection  he  desires  especially  to  thank  Dr 
J.  C.  Watt  for  much  generous  assistance  in  the  revision  of  the  manu- 
script and  for  undertaking  the  correction  of  the  proof-sheets. 

In  addition  to  the  works  mentioned  in  the  preface  to  the  first 
edition  as  of  special  value  to  the  student  of  Embryology,  mention 
should  be  made  of  the  Ha„4buck  der  vergleichef.dm  und  experimen- 
mm  EntunckhmgsUhre  der  Jtrirbatiere  edited  }iy  Professor  Oscar 
Hertwig  and  especiaUy  of  the  Manual  of  Human  Embr.Mgy  edited 
by  Professors  F.  Keibel  and  F.  P.  Mall. 
UNiYixsiry  o»  Tobonio. 


PREFACE  TO  THE  FIRST  EDITION. 


The  assimilation  of  the  enormous  mass  of  facts  which  constitute 
what  s  usually  known  as  descriptive  anatomy  has  always  been' 
dtfficult  task  for  the  student.  Part  of  the  dimJulty  hasZn^e  to 
a  lack  of  mformation  regarding  the  causes  which  have  dnermined 

knowledge  of  the  why  thmgs  are  so.  the  fact-  ,f  anatomy  sUnd  as  so 
m«y  «o  ated  .terns,  while  with  such  know'     ge  they  bLme  bol" 

TtZl"  "  "'""  '"'  '""^  """^  '''^"'»"  ••>«  <''«ni'y 

The  g^t  key  to  the  significance  of  the  structure  and     -lations 
«l^^t^  H^-f '',T*'"^°«"'^'"«^y'''»"«'»»''e  ■    toSa 
M  well  a,  the  mdrndual  development,  and  the  following  pages  con 
smute  an  attempt  to  present  acondse  statement  of  the  development 
of  thehunun  body  and  a  foundation  for  the  proper  understanding" 

tJelT     h'"*T-    N«t.«illy.  the  individual  development  cWms 
the  major  share  of  attention,  since  its  processes  a«  the  Lre  immed^^- 

ZiZ  ">«  •'■"Wolog.cal  record  fuls  to  afford  the  required  data 
whether  from  Us  actual  imperfecUon  or  from  the  incompleteness 
of  our  knowledge  concerning  it.  recourse  has  been  had  toX  fa«s  o 
compa^fve  anatomy  as  affording  indications  of  the  hisToriTal  dtel 
opment  or  evolution  of  the  parts  under  consideration 
of  fl,*  '■*' ""'.^"""'"J  f«»«iWe  to  include  in  the  book  a  complete  list 

rai  !J'^  ~'""^"''  '""  "^  P'^P"''''°»-    The  short  Siog 

raphies  appended  to  each  chapter  make  no  pretensions  tar^^ 
pleteness.  but  are  merely  indications  of  some  ofTm    elmpoZ" 
works,  especially  those  of  recent  date,  which  consider  the  ZSn 
discussed.    For  a  very  full  bibliography  of  all  works  trlttag  of 

vii 


viii 


PBEFACE   TO   THE   FIBST   EDITION 


of  the^o^^i^^S^f^fS^K?''  P"/'"'""'  ^  *"  "Memoir, 
fitting.  howeve^:l2.^SS  ^S^JtrbtT^^^ '*  " 
by  all  ^ters  on  humn  embr4C  TZ^.r  •  '^'*'  '''^ 
chief  among  which  is  his  A^l^'  *' f'*^"=  PaP«"  of  His. 
grateful  ackiowIedJents  .iT^  '^Micher  Embryonen.  and 

Aittioinou,  Labomtosy, 
UNivMsiry  or  MicmoAjf. 


CONTENTS 

iKIKODCCnON    .    .    .  ''"' 
I 

PART  I.— GENERAL  DEVELOPMENT. 

CHAPTER  I. 

The  Spermatozoon  and  Spermatogenesis;  the  Ovuii  and  Its  Matu- 
ration and  Fertilization 

II 

CHAPTER  II. 

The  Segmentation  of  the  Ovum  and  the  Formation  of  the  Germ 
layers     .... 

38 

CHAPTER  III. 

The  MeduUary  Groove,  Notochord,  and  Mesodermic  Somites  .   .   .     64 
CHAPTER  IV. 

The  Development  of  the  External  Form  of  the  Human  Embryo.   .        86 
CHAPTER  V. 

The  Yolk-stalk,  BeUy-stalk,  and  Fetal  Membranes 107 

PART  II.-ORGAN06ENY. 
CHAPTER  VI. 

The  Development  of  the  Integumentary  System ,^, 

CHAPTER  VII. 

The  Development  of  the  Connective  Tissues  and  Skeleton    ....   153 
CHAPTER  VIII. 

The  Development  of  the  Muscular  System ,., 

iz 


CONTENTS 


CHAPTER  DC.  Pao. 

.  CHAPTER  X. 

The  Develop^em  of  .he  Digestive  Tract  and  Glands 

CHAPTER  XI. 
The  Development  of  the  Pericardium, 


a8o 


the  Diaphragm 


the  Pleuro-peritoneum,  and 


The 


CHAPTER  XII. 

Development  of  the  Organs  of  Respiratio 
CHAPTER  XIII 
The  Development  of  the  Urinogenital  System. 

CHAPTER  XIV. 
The  Suprarenal  System  of  Organs 


CHAPTER  XV. 
The  Development  of  the  Nervous  System 

CHAPTER  XVI 

The  Development  of  the  Organs  of  Special  Sense. 

CHAPTER  XVII. 
Post-natal  Development 
Index  . 


316 


Sli 


338 


370 


•  377 

•  4*7 

470 
487 


THE  DEVELOPMENT 

OF  THE 

HUMAN  BODY. 


INTRODUCTION. 

Somewhat  more  than  seventy  years  ago  (1839)  one  of  the  funda- 
mental prmciples  of  biology  was  established  by  Schleiden  and 
Schwann  as  the  cell  theory.    According  to  this,  a'l  organfms  are 
composed  of  one  or  more  structm-al  units  termed  ceUs,  each  of  which 
m  multicellular  organisms,  maintains  an  individua   existenc™' 

vidual.    Viewed  m  the  light  of  this  theory,  the  human  body  is  a 

I  leTr^'  ''I  ^■''''"'''  °'  ""°^  ^•"^^-'  units,"ach  of  wHch 
leads  to  a  certam  extent  an  independent  existence  and  yet  both 
con^ibutes  to  and  shares  in  the  general  welfare  of  the  con^l^U^' 

wi  Jh  T-°  r  °^  '^'  ^''^•"^  *^  ='™«"'-l  "nits  were  vesicles 
w.th  definite  walls,  and  little  attention  was  paid  to  their  contenVs 
Hence  he  use  of  the  term  "cell"  in  comiectL  with  them  Lotg 
before  the  establishment  of  the  cell  theory,  however,  the  exisrencf 

.io„?r'T/°T"'  "' '  «^'^^^°"^  -''^t-"  sho;ing  no  Sa! 
tions  of  a  definite  limiting  membrane  had  been  noted,  and  in  rZ\ 

ofTh^ch^^r  fn'm"!"^-^^''"'  '-'  ''-'''''  '''  gelat'lnous^tel! 
t^Xs^clTr"""'""'""'  (Rhizopoda)  were  composed. 
S  .  Z""^Ti  '"^'"t^'n'ng  it  'o  be  the  material  substratum 
which  conditioned  the  various  vital  phenomena  exhibited  bv  Th! 

iXr-t  "-r-  "  '''"'  "  *'°''"'''*'  -"  Mohl,  obsSed'ti 
l.vmg  plant  cells  conUmed  a  similar  substance,  upon  which  he 


*  INTRODUCTION 

believed  the  existence  of  the  cell  as  a  vital  structure  was  dependent, 
and  he  bestowed  upon  this  substanc(!  the  name  protoplasm,  by  which 
It  is  now  universally  known.  •■      r'      ,   }  wnicn 

By  these  discoveries  the  importance  originally  attiibuted  to  the 
cell-wall  was  ^eatly  lessened,  and  in  1864  Max  Schultze  reformu- 
lated the  cell  theory,  defining  the  cell  as  a  mass  of  protoplasm,  the 
presence  or  absence  of  a  limiting  membrane  or  cell-wall  being 
immatenal.  At  the  same  time  the  spontaneous  origination  of  cells 
from  an  undifferentiated  matrix,  believed  to  occur  by  the  older 
authors,  was  sho^~  to  have  no  existence,  every  cell  originating  by 
the  division  of  a  preexisting  cell,  a  fact  concisely  expressed  in  the 
aphonsm  of  Virchow— <»>»»»>  ceUula  a  ceUuld. 

Interpreted  in  the  light  of  these  results,  the  human  bodv  is  an 
aggregate  of  myriads  of  cells,*-,-.  ,.,  of  masses  of  protoplasm,  each 
of  which  owes  Its  ongm  to  the  divbion  of  a  pregxistent  cell  and  all  of 
which  may  be  traced  back  to  a  single  parent  cell-a  fertilized  ovum. 
All  these  cells  are  not  alike,  however,  but  Justasin  a  social  community 
one  group  of  mdmduals  devotes  itself  to  the  performance  of  one  of 
the  duties  requisite  to  the  well-being  of  the  community  and  another 
group  devotes  itself  to  the  performance  of  another  duty,  so  too 
m  the  body    one  group  of  ceUs  takes  upon  itself  one  special' 
function  and  another   another.    There    is,  in  other  word^  in 
the  cell-commumty  a  physiological    division  of  labor.    Indeed 
the  comparison  of  the  cell-community  to  the  social  community  may' 
be  carried  still  further,  for  just  as  gradations  of  individuality  may  be 
recognized  m  the  individual,  the  municipality,  and  the  state,  so  too 
m  the  cell-community  there  are  cells;  tissues,  each  of  which  is  an 
aggregate  of  similar  cells;  organs,  which  are  aggregates  c<  'issues,  one 
however,  predommating  and  determining  the  character  of  the  organ^ 

f'unctS!"'  """^  '''  ''^'^"*''  °^  °'S"°^  •'^^^S  '^''-^''ted 
It  is  the  province  of  embryology  to  study  the  mode  of  division  of 

II  has  been  eslimated  that  the  number  of  cells  ent^rl-.  i-.„  .1. 
lHe_^.  .  an  a.„.t  hnn^  beh,«  .  abont  ..^^^^'^T.t^rC^^ 


Fio.  I. — Oruii  OP  Nzw-BOKN 
Child  whb  Follicle-celis. — 
(Mttem.) 


INTKODtrcnON  , 

the  fertUized  ovum  and  the  progressive  differentiation  of  the  resulting 
cells  to  form  the  tissues,  organs,  and  systems.  But  before  consider- 
mg  these  phenomena  as  seen  in  the  human  body  it  will  be  well  to  get 
some  general  idea  of  the  structure  of  an  animal  celi. 

This  (Fig.  I),  as  has  been  already  stated,  is  a  mass  of  protoplasm, 
a  substance  which  in  the  living  condition  is  a  viscous  fluid  resembling 
m  many  of  its  peculiarities  egg-albumen,  and  like  this  being  coagu- 
lated when  heated  or  when  exposed  to 
the  action  of  various  chemical  reagents. 
As  to  the  structure  of  living  protoplasm 
little  is  yet  known,  since  the  application 
of  the  reagents  necessary  for  its  accurate 
study  and  analysis  results  in  its  disin- 
tegration or  coagulation.  But  even  in 
the  living  cell  it  can  be  se»n  that  the 
protoplasm  is  not  a  simple  homogeneous 
substance.  What  is  termed  a  nucleus  is 
usually  clearly  discernible  as  a  more  or  less  spherical  body  of  a 
^ater  refractive  index  than  the  surrounding  protoplasm,  and  since 
this  IS  a  permanent  organ  of  the  cell  it  is  convenient  to  distinguish 
the  surroundmg  protoplasm  as  the  cytoplasm  from  the  nuclear 
protoplasm  or  karyoplasm. 

The  study  of  protoplasm  coagulated  by  reagents  seems  to  indi- 
cate that  It  IS  a  mixture  of  substances  rather  than  a  simple  chemica' 
compound.  Both  the  cytoplasm  and  the  karyoplasm  consist  of  a 
more  solid  substance,  the  reliculum,  which  forms  a  network  or  felt- 
work,  m  the  interstices  of  which  is  a  more  fluid  material,  the  end,y- 
lema*  The  karyoplasm,  in  addition,  has  scattered  along  the  fibers 
of  Its  reticulum  a  peculiar  material  termed  chromatin  and  usually 
contams  embedded  in  its  substance  one  or  more  spherical  bodies 

i««l.\'Tk'^  "'■""'^  '^'  "^"  coagulable  substance,  and  gelatin,  when  sub- 
™te  ,0  Uiat  of  protoplasm,  and  it  has  been  held  that  pretoplasm  in  the  uncoagulated 
oC  w'^il^.T  """^f^"'  '  "°"  "  '^  boLg^eous  material  X^ 


A 

INTRODOCnON 

growing  cdls  there  is  differenced  Lthe  XSa  ^it  d' 
known  as  the  archoplasn,  sphere,  in  the  center  ^f  wSl^t 
usually  a  minute  spherical  body  known  as  the "^^tj^'  ^"'^  " 

plicated  phenomena  which  have  ilat  ZS  """"  °'  '"'"- 

some  of  the  problems  of  embr;:^  l^en  sucl':  'cT t\'^*' 
described  above  is  about  to  divi^  thrfibers  of  tH       -'^        " 

theneighborhoodofthearchoplasmspLa'Llt--.'    "^ 
to  form  fibrils  radiating  in  all  directions  frnmT^!"'  "^  "' 

(Fig.  .,  A).  andlTtertli^s  t'hSIfbrtaks  rmtVa°HT°"  *'""' 

of  pieces  termed  chrof^sones  (^J      B)  ir^t ''^T  "T'"' 

.  practically  constant  for  ea^h  specfes  ^f  aiim!,  T''"  °*  *^  "^^^ 

has  been  placedattwenty-fourTne^^^'esKrK" ''!""'"''" 
observations  of  Guyer  ■I^cJZ^^l^S^.i:^^^^-^^^ 
female  and  tiventy-two  in  the  male  T»,„  =;„  -c  '"^^'y-'""  "» the 
ence  in  the  two  sexes  wilbrmn.-HV^'^'""'^' °^  ^'^ '^^«- 
fertilizationof  thermcp.';     ""'"'  ""  """"'°"  ^'"^  '•«= 

As  soon  as  the  asters  have  tAlrpn  i,t,  n.  ■ 
fides  of  the  nucleus,  the  ^^^^^Z^ZZZCc  "'^f^ 
mto  a  spindle-shaped  bundle  of  fibrils  wh.V),  •        converted 


INTBODUCnON  . 

themselves  in  a  circle  or  plane  at  the  equator  of  the  spindle  (Fig  2  D) 
and  the  stages  preparatory  to  the  actual  division,  the  prophases,  are 
completed.  "^         ' 

The  next  stage,  themetaphase  (Fig.  3,  A),  consists  of  the  division, 
usually  longitudinally,  of  eaS^chromosome,  so  that  the  cell  now 


F.G.  ,.-DUG»AlC  iLLCSTMTmo  ^  PROFaiSES  OF  MrrosB.-{.4 rf«^rf/r.„ 
i.  0.  Wtlson.) 

contains  twice  as  many  chromosomes  as  it  did  previously.  As  soon 
as  this  dmsion  is  completed  the  anaphases  are  inaugurated  by  the 
-s^  of  each  chromosome  separating  from  one  another  and  ap- 
proachmg  one  of  the  asters  (Fig.  3,  B),  and  a  group  of  chromosomes, 
contammg  half  the  total  number  formed  in  the  metaphase,  comes  to 


6 

IMTXOOUCnON 


cytoplasxa  (Fig.  3,  C)  anT^f  Wrance  at  the  surface  of  the 
plasm  ta  aplaLt^i  "thrf^w.^  '^"'^'^^'  '^^'^'^  '^e  cyto- 


iMTsooDcnoii  - 

TW.  complicated  process,  which  is  known  as  karyokinesis  or 
»..to5«,u  the  one  usually  observed  in  dividing  cells,  but  occasionally 
a  cell  divides  by  the  nucleus  becoming  constricted  and  dividing  into 
wo  parts  without  any  development  of  chromosomes,  spindle,  etc 
he  division  of  the  cell  following  that  of  the  nucleus.    This  aml^ 
totic  method  of  division  is.  however,  rare,  and  in  many  cases,  though 
not  always,  itsoccurrence  s-.ms  to  be  associated  with  an  Impairment 
of  the  reproductive  activities  of  the  cells.    In  actively  reproducing 
cells  the  mitotic  method  of  division  may  be  regarded  as  the  rule 
Smce  the  process  of  development  consists  of  the  multiplication  of 
a  smgle  ongjnal  cell  and  the  differentiation  of  the  cell  aggregate  so 
formed,  it  follows  that  the  starting-point  of  each  line  of  individual 
development  is  to  be  found  in  a  cell  which  forms  part  of  an  individual 
of  the  precedmg  generation.    In  other   words,  each   individual 
represents  one  generation  in  .sse  and  the  succeeding  generation  m 
posse.    This  Idea  may  perhaps  be  made  clear  by  the  following  con- 
siderations.   As  a  result  of  the  division  of  a  fertilized  ovum  there  is 
produced  an  aggregate  of  cells,  which,  by  the  physiological  division  of 
abor.  specialize  themselves  for  various  functions.    Some  assume 
the  duty  o   perpetuating  the  species  and  are  known  as  the  sexual 
or  germ  cells,  while  the  remaining  ones  divide  among  themselves  the 
various  functions  necessary  for  the  maintenance  of  the  individual 
and  may  be  termed  the  somatic  cells.    The  germ  cells  represent 
potentially  the  nert  generation,  while  the  somatic  cells  constihile  the 
present  one.    The  idea  may  be  represented  schematically  thus: 
Fint  generation 


Somatic  cells  -i-  germ  cells 


aecond  generation 
Somatic  cells  +  gena  cells 
Third  generation 


Somatic  cells  +  germ  cells,  etc. 

It  is  evident,  then,  while  the  somatic  cells  of  each  generation  die 
at  then-  appomted  time  and  are  differentiated  anew  for  each  genera- 


8 

iNnooucnoN 

«w'w.  This  is  the  doctrin;  of  ^  ^  "'  **'  ^»  »>>"  ««  «4 
docrtne  of  fundamenunrpl'^tr"''"  "-^  ""  '--^'-.  • 
the  phenomena  of  heredity  '"""*  "'  "'  '^'^8»  on 

of  each  individual,  and^h^s  ,  ^  TS^fil^^'f  '"'  "'^'"'P'"™* 
»eant  a  genn  cell  produced  by  the  tiinfr"  """'"•  ^^  ""i.  i, 
plasm.  In  nwny  of  the  lower  f™™  T.  "'^  """''  of  the  germ- 
turbelJarian  worms) «prSu  ion! "li'^"  ^'•*'  •^>*'' «»<•  cSn 
of  the  entire  organisS^tw'LTo'r  Sir"''"'''''  ^"^  "  '«'^'»'' 
of  the  body  fr^m  the  parent  Sdulf't''r"'*'°°°'*P°^°" 
duction  is  termed  ««.^Lj    p"  h?  •'  "  '^*''*^  »*  «P«>- 

('•^..  bees, Phylloxera,  wSea^UheT"  'n  '  """"^'  <"  'orms 
development  without  pr^SusW  il    ^^  ""^  '"*  "We  to  undergo 

■  «;^ method  of  «producS  Swi^r/iSi:''  ''^'  '^°"'*''"''°« 
these  cases  .«»«a/  reproduction  alZ.^  ^«*f^^«««w.  But  in  all 
organized  animals  it  is  Sy  metS  T  ""  "'  *''*  """^  highly 
germ  cell  develops  only  aft«  =0^,..  w  ^^  """*"y  °^^'  ^  it  a 
I»  the  simpler  f^  Ll^p^t  ^T-;'"' '-*«  ^e™  cell, 
the  two  combining  cells  but  si^T  •  ^^"^""^  "^^  '>«''«'> 
a  certain  amount  of  nutritfolT  1 1'  "^  "  "•'''  °^  "''^'^tage  that 
forthesupport  of  the  deSZ ?e  mb„    ^'"^  ^  ''^  '""^  «-«  <=«" 

for  itself,  while  at  the  sar2ftTsaT^T"'''''''''''*''«f°od 
which  unite  shall  come  fromdSi't  f^  -^T^^'""' *"  the  cells 

«>d  hence  that  the  cells  slufH     '^r'^"*''  («os^fertilization). 

labor  has  resulted.    cLt   '    " '""  '^'"  "°''«ty.  »  division  o 

yolk,  their  motility  bSmZShv      ''""J"'  ""^  °'  '^^  f°od 

termed  the  female  cells  oTZ  wMeotrT''  ?'  '°™  "''''*  «- 

storing  up  nutrition,  are  eT^cMv"  .?'''?"' '"P""'"^°"«  of 

trate  the  inert  ova;  these  K^IU         ""*  '""  ^^^  ^°<1  P^"*" 

spermato^a.    In  many  adma"   bo  J  VT","*'  '""^  "^'^  «"^  ^r 

--..wdu.,hL.rti^^-:sn^^^ 


umoDncnoN  . 

in  some  of  the  lower  order.)  each  individu.1  produces  only  ova  or 
spermatOHia,  or.  as  it  is  generally  stated,  the  sexes  are  distinct. 
It  U  of  importance,  then,  that  the  peculUrities  of  the  two  forms 
of  germ  cells,  as  they  occur  in  the  human  species,  should  be  con- 
sidered. 


LITERATURE. 

^■"yoa!^,^.'"^^'"'"'""^'^""  "'""'■"""«•"    Third  «li.i«.    N,, 
O.  Heitwlg:  "Die  Zdle  und  die  Gewebe."    J,n»,  1893. 


PART  I. 
GENERAL  DEVELOPMENT. 


CHAPTER  I. 

THB   SMRMATOZOOir  AOT)   SPERMATOGEHBSIS:    THE 

OVUM  AHD  ITS  MATURATIOH  AlTD 

FBRTILIZATIOir. 

The  Si>enn«toio»n.-The  human  spermatozoSn  (Figs.  4  and  s) 
.s  a  imnute  and  greatly  elongated  cell,  measuring  about  0.05  mm  in 
length.    It  consfats  of  an  anterior  broader  portion  or  head  (Fig  <  H) 
which  measures  about  0.005  mm.  in  length  and,  when  viewe  1  :Vom' 
one  sitfface  (Fig.  4, 1),  has  an  oval  outline,  thoug.'.  since  it  U  some- 
what flattened  or  concave  toward  the  Up,  it  has  a  pyrifcrm  shape 
when  seen  m  profile  (Fig.  4,  a).    Covering  the  flattened  portion  of 
the  head  and  fitting  closely  to  it  is  a  delicate  cap-like  membrane, 
the  head-cap  (Fig.  5,  He),  whose  apex  is  a  sharp  edge,  this  structure 
correspondmg  to  a  pointed  prolongation  of  the  cap  found  in  the 
spermatozoen  of  many  of  the  lower  vertebrates  and  known  as  the 
perforatorium.    Immediately  behind  the  head  is  a  short  portion 
known  as  the  neck  (Fig.  5,  N),  which  consists  of  an  upper  more 
refractive  body,  the  anterior  nodule,  and  a  lower  clearer  portion 
To  this  succeeds  the  cofmectmg  or  middle-piece  (Figs.  4  and  3,  m) 
which  begms  with  a  posterior  nodule,  from  the  center  of  which  there 
pa^s  back  through  the  axis  of  the  piece  an  axial  filament,  enclosed 
withm  a  sheath,  this  latter  having  wrapped  around  it  a  spiral  fila- 
ment.   At  tht  lower  end  of  the  middle-piece  this  spiral  filament 
terminates  m  the  annulus,  through  which  the  axial  filament  and  its 
sheath  passes  into  tht  flagdlum  or  taU  (Fig.  4,  J).    This  portion 


la 


THE   SBEKMATOZOON 


Which  constitutes  about  four-fifths  of  th.  .  .  i  ■ 

•natozoen  is  composed  simX  o  the    '    >  «°.  '^^  °^  '^'  '^'■ 

this  latter  gradual^  thinnhg  out  as  t  "t      t^^T  '""'  '>  ^^"'h, 

-o.ethera  shon  -^^^Z:' ^jz'^rt:':^:^^^ 


(Fig.  4,  .).  °''°  "'  "'^  '«mf^;^/a«,«  or  end-fiece 

the  cCJroVttteSr  °'  ''^  ^~  ''^-  --'"« ^«° 

is  necessary,  and  sin  e  thTt^*  f"     "  ''"''^  "'  ""^^  development 
^ee«.uchmoreaccu::^Xi:^--^^Have 


SFESHATOGEMXSIS 


13 


and  guinea-pig  tiian  in  man,  the  description  wliich  follows  will  be 

I  based  on  what  has  been  described  as  occurring  in  these  forms. 

From  what  is  known  of  the  spermatogenesis  in  man  it  seems  certain 

that  it  closely  resembles  that  of  these  mammals  so  far  as  its  essential 

I  features  are  concerned. 

Spermatogenesis.— The  spermatozoa  are  developed  from  the 
cells  which  line  the  interior  of  the  seminiferous  tubules  of  the  testis. 
The  various  stages  of  development  cannot  all  be  seen  at  any  one 
part  of  a  tubule,  but  the  formation  of  the  spermatozoa  seems  to  pass 


FlO.   6.— DUOKAK  SHOWING  STAGES  OF   SpERMATOOENESIS  AS  SEEN   I.N   DlprEMNI 

Sectoks  or  A  Seiiinifekous  Tubule  of  a  Rat. 
i,  SertoU  cell;  sc',  spermatocyte  of  the  Brst  order;  sc',  spermatocyte  of  the  second 
order:  j^    spermatogone;  sp,   spermatid;   sz,  spermatozofln.— (AfmJiVfei  from  von 
Lenhossek.)  "^  ■' 


along  each  tubule  in  a  wave-like  manner  and  the  appearances  pre- 
sented at  different  points  of  the  wave  may  be  represented  diagram- 
matically  as  in  Fig.  6. 

In  the  first  section  of  this  figure  four  different  generations  of 
cells  are  represented;  above  are  mature  spermatozoa  lying  in  the 
lumen  of  the  tubule,  while  next  the  basement  membrane  is  a  series 
of  cells  from  which  a  new  generation  of  spermatozoa  is  about  to 
develop.    The  cells  of  this  series  are  of  two  kinds;  the  larger  one  (i) 


H 


SPEMtATOCENESIS 


In  the  next  section  the  SertolH^r-  "^"^  'P^^natogonia  (s.) 

abJenlarged.itscytoplS^jSn';;:- «°  DfT  Worn/ consiS 
and  n.  the  third  section  the  e^Sem  hi,     '""'""^  *'''''»''«'«• 
e^ent  that  the  spern>atogonia  ^TZ\        '°f '"'^  *°  ^"^^  an 
membrane,  with  which  the  S^rt^   e  ^17'"  •'™°'  *'"''  '«'-»«* 
fourth  section  the  spernutogoS  a '  ,t        "  '"  *^°"'"=*-    I«  'he 
one  of  the  cells  so  formed  .^ZJTt,T  ""  '^°''''  "'  '^^'O'^' 
"ther  forms  what  is  termed  a  Mwl    r"^*°«°''^' ^^ile  the 
results  of  the  division  are  seenl^T^  'P^fnalocyie  (sc').    The 
«atogonia  are  seen  a«S^  „  "  °  *'""  '*''  =*==«°°.  ^here  four  sp^ 
and  above  them  ISZ^'^I  "''''  ''''  "--^nt  membr^'e 
^the  fet  and  second  Z^^Z:^:^^''''-    ^^"-^«  °- 
niay  sWI  be  seen,  indications  of    '        °  ^""^ 'P«™a'°cytes 
furnished  by  the  an^ngS  o    fheT°*'""«  '""■«'°»  ^ig 
second  se^ion,  and  in  theTS  Lc^tn  th^T-'^"  ■"  «"'-  °^  «>' 
«^.  the  two  c.:.,  which  r.sult^^tTr'°''''^^ninpro,. 
spermatocytes    (sc').    These    ^L     ?       ^^""^  *^™^d  ^econth^y 
division,  as  shown  in  the  fou^t^"""''    '""nediate.y    u„^ 
;^«  (SP),  each  of  JS  wl?  T'  ^^«  ^  *»  tw^ 

;=arb:e=-:£^=^^^^^^ 

'n  the  somatic  cells.  The  gJ^rlfiT  t"  '^'  """^'^  occurring 
-ay  be  described  as  follo^  fo  ^4"'"'^ ^"^ '^  ^'^^"-P'-^^^^ 
he  number  of  chromosomes  that  ann/°^  of  the  spermatogonia 
">  the  somatic  cells,  so  that  L^formTos"  "'"*'"'  "^*''  *'''"  ^^^ 
eight  chromosomes  appearTn  each  ,1  ""^"'  """"^^  ■=  eight, 
Jat  eight  pass  to  ea^^  of  the  rtsj^~'°°""'  ''"'^ ''-"'«' 
When  these  cells  divide,  however  7h!!\^"'^'^  spermatocytes. 


SPESUATOCENESIS 


IS 


supposed  case  that  is  bemg  described  (Fig.  7,  sc').  The  further 
history  of  these  chromosomes  indicates  that  each  is  composed  of 
four  elements  more  or  less  closely  united  to  form  a  tetrad,  and  during 
I  mitosis  each  tetrad  divides  into  two  dyads,  four  of  which  will  there- 
fore pass  into  each  secondary  spermatocyte.    These  cells  (Fig.  7,  sc') 


/        \ 


Fig.  7.-D1AGEAM  iLLUSTRATmc  THE  Reduction  of  the  Chromosomes  During 

Spermatogenesis. 

«'.  Spermatocyte  of  the  first  order;  «».  spermatocyte  of  the  second  order-  sp 

spermatid. 

undergo  division  without  the  usual  reconstruction  of  the  nucleus  and 
each  of  the  dyads  which  they  contain  is  halved,  so  that  each  sper- 
matid receives  a  number  of  single  chromosomes  equal  to  half  the 
number  characteristic  for  the  species  (Fig.  7,  sp). 

This  account  of  the  behavior  of  the  chromosomes  during  sper- 


i6 


SPEWttTOOENMIS 


Juatogenesis  assumes  that  all  f»,«  ,1, 
spermatocytes  are  of  equal  JS  aStr"  °'  *■=  ^^^ 
mitosis.    It  has  been  found  however  1^      '  '"^''^^  ''"^8 
Onsects,  spiders,  birds,  etc.,)  tWsTp-^-  tt   ""  "  """''^'-  °^  ^"^^ 
vation,  by  Guyer  indicate  thaS  ^■'«minf  tTe   '""*  "''''■ 
chromosomes  differ  decidedly  fromtheirlT      '''^/P^'^'ocytic 
of  the  primary  spermatocytes  twelve  c^lr'    ^*  *«> '»'-'<"' 
appearance,  but  two  of  these  differ  ft„^  chromosomes  make  their 
divide,  but  pass  directly  to  onf^  '7 '*;'  "f  ?  "''''  *••«>  ^o  »ot 
(Fig.  8).    When  the  di^^on  is  com^l  ^^  '  °'  ''''  ""''""^  ^P^^dle 
two  daughter  secondary  sLrmtS'  T^""^^''  °'"^  "'  '^^ 
undivided  or  a.cm«,/Z«^ZT!  "^^  "^^'^  '""> 

resulting  from  the  diil^Tt:*;"  ^''"'^ ''"°'"°-'--' 
chromosomes;  the  other  dWhter  cell  on  t\^.?"^.  ^permatocytic 
received  only  ten  ordinarv  1  '       ^^^  '"^*»'  '"^d,  will  have 

secondary  s;rSyre~erta^""°r*''-'''--' 
possess  twelve  chromoUesanSt^triyter'  the  cells 

.enXiTErSirrdr?-^^^^^^^^ 

spermatocytes  reveals  a  seco«*\,t"T  °'  *1'  "•^^^'^^^^ 

.nsteadonwelveandtenchromoslrseLardfi"      "'  '^*°"^ 
make  their  appearance     ThU  ™o  T       T       ^  ^''^'  respectively, 

thatthetenoEycLm„tL?'  ''^^^P^r^don  the  supposition 

spermatocytes,  harSrtnorr'v"^'^^^"'^^'^-*''^ 
while  the  two  accessor,  ctomosoZ  ^''''°*  chromosomes, 

have  remained  distinct    D^  the  ^f  ■'!'"  °°^  °^  '^^  «='-ses 
somes  divide  just  as  do  the  o^fn  '  ^^'  ^''''''''^  '^'>romo- 

tocyte  of  one  class  two  s^^^tEeT  V"^'  ^°"  ^"'^  =P«^«- 
chromosomes,  while  from  ^h  ™''*'  ''"^'^  '""'''^g  seven 

spermatid.,  e;ch  tn^n  "g  Vvfrr^*'  °'  "^^  ^'^'^^  <='-  *- 
Since  the  spermatids  are  dLctly  SZ'^V-  ''''^'  ^*  «>• 
half  of  these  latter  will  have'c  LdT^^^^^^  "'°  spermatozoa, 

sP^rmato^aofoneclarrrr-S--^- 


SPERMATOGENESIS  j- 

Iten  plus  two,  i.  e.,  twelve  univalent  chromosomes,  while  those  of 
Ithe  other  class  will  have  received  the  equivalent  of  only  ten  * 
I  .  The  transformation  of  the  spermatids  into  spermatozoa  takes 
lp.ace  while  they  are  m  intimate  association  with  the  Sertoli  cells 
la  number  of  them  fusing  with  the  cytoplasm  of  an  enlarged  Sertoli 
I  cell,  as  shown  m  Fig.  6,  s,  and  probably  receiving  nutrition  from  it. 
In  each  spermatid  there  is  present  in  addition  to  the  nucleus,  an 


'  t^etl''"!  '^^''"T^  *^°  centrosomes  that  have  migrated  from 

he  archoplasm  and  he  free  in  the  cytoplasm.    The  centrosome^ 

^  and  the  archoplasm  sphere  take  up  their  position  at  opposite  poles 

|Ofthenucleus,thearchoplasmeventuallvformingthehead-capofthe 
I  spermatozoon,  and  from  one  of  the  ..ntrosomes  a  slender  axial 

them  into  .^'  Zl  air,  „K  !r"'  ""  t'  ^"^^^^^  »'  ^-  spermatids  dividing 
by  our  ^,  t:^t        <"  ""''  "=  '""'"^"''  """^^^^  indistinguisi^abie 


i8 


SPEUIATOGENZSIS 


aament  grows  out  and  soon  projects  beyond  the  limits  of  the  cyto- 
plasm (Fig.  9,  A).  The  other  centrosome  becomes  a  rod-shaped 
structure  which  applies  itself  closely  to  the  posterior  pote  of  Ae 
nucleus  becoming  the  anterior  nodule,  while  the  lower  one,  from 
which  the  filament  arises,  becomes  at  first  pyramid- '  in  shape 
(*ig.  9,  B)  and  later  separates  into  a  rod-Uke  portion  to  which  the 
lament  is  attached  and  a  ring,  through  which  the  filament  passes 
^*ig-  9.  C).    The  rod-like  portion  becomes  the  posterior  nodule 


Fio.  9.-B1AOZS  m  iHi  TiASsrosHATioN  OF  A  Spkhhatib  into  t 
Speehatoioon.— (^//er  Ueves.) 

and  the  ring  separates  from  it  to  form  the  annulus  (Fig  9  D)  The 
nucleus  becomes  the  head  of  the  spermatozoon,  the  cytoplasm  sur- 
rounding it  becoming  reduced  to  an  exceedingly  delicate  layer,  so 
that  the  head  is  composed  almost  entirely  of  nuclear  substance  if 
the  heaa-cap  be  left  out  of  consideration.  The  spiral  filament  of 
the  middle-piece  is,  however,  a  derivative  of  the  cytoplasm  and 
according  to  some  authors  this  portion  of  the  spermatid  also  fur- 
mshes  the  material  for  the  sheath  of  the  axial  filament,  though 
this  has  been  denied  (Meves),  the  sheath  being  regarded  as  a  differ- 
entiation of  the  axial  filament.  Each  spermatozoon  is,  then,  one 
of  four  equivalent  cells,  produced  by  two  successive  divisions  of  a 

primary  spermatocyte  and  containing  one-half  the  number  of  chromo- 
somes characteristic  for  the  species. 


THE   OVUh 


19 


The  number  of  spermatozoa  produced  during  the  lifetime  of  a 

■  single  individual  is  very  large.  It  has  been  found  that  1  cu.  mm.  of 
I  human  ejaculate  contains  60,876  spermatozor,  a  single  ejaculate 
Itherefore,  containing  over  200,000,000.  This  would  indicate  that 
Idurmg  his  lifetime  a  man  may  produce  340  billion  spermatozoa 
I  (Lode). 

The  Ovum.-The  human  ovum  is  a  spherical  cell  measuring 

■  aljout  0.2  mm.  m  diameter  and  is  contained  within  a  cavity  situated 


.  Mood.ves«l,  dp,  duKu,  prohgerus;  mf  str^um  panuloaum;  „,  ovum;  ..  stroma; 
tn,  tneca  folliruli. 

It?'  rirV"'  '"^^"  °*  *'''  "^"y  ^■'d  t^™^d  a  Graafian  foUide. 
I  h.s  follicle  IS  surrounded  by  a  capsule  composed  of  two  layers,  an 
■outer  one,  the  theca  externa,  consisting  of  fibrous  tissue  resembling 
■that  found  m  the  ovarian  stroma,  and  an  inner  one,  the  tkeca  interna. 
icomposed  of  numerous  spherical  and  fusiform  cells.    Both  the 


30 


THE   OVCIf 


theoB  are  richly  supplied  with  blood-vessels,  the  theca  interna 

to  the  theca  mtema  there  is  a  transparent,  thin,  and  structui^ 
hyaline  membrane,  within  which  is  the  follicle  proper,  who^" 

ormedbyalayerof«,ls  termed  the  .Va/„.JXjXxtii 
and  .nclosmg  a  canty  filled  with  an  albuminous  fluid,  L  liJL 


Fio.  II.-Ov™  FEOII  OVARV  OF  A  WoiIAM  THKTY  YeAM  OF  AOE 

c.  Corona  rad,aU;  „,  nuclemj  A  protoplasmic  «,ne  of  ovum;  J,,  periviteUin.  space 
y,  yolk;  ,A  iona  pellucida.— (iVa^rf.)    ^  '  •"""«""■'  '?»<:«■ 

follkuli.  At  one  point,  usually  on  the  surface  nearest  the  center 
Of  the  ovary  the  stratum  granulosum  is  greatly  thickened  to  form  a 

cattv  f.'l!  f'n  ?  *'""  ^"''^"""^  (''^^'  ^•''^•>  P™J«t^  i°to  the 
cavity  of  the  follicle  and  encloses  the  ovum  (o).  Usually  but  a  single 
ovum  IS  contained  in  any  discus,  though  occasionally  two  or  even 
tfiree  may  occur. 


OVULATION  AND   THE   CORPUS   tUTECM  aj 

The  cells  of  the  discus  proligerus  are  for  the  most  part  more  or 
less  sphencal  or  ovo.d  m  shape  and  are  arranged  irregularly.  In 
the  immediate  vicmity  of  the  ovum,  however,  they  are  more  columnar 
m  form  and  are  arranged  in  about  two  concentric  rows,  thus  giving 
a  somewhat  radiated  appearance  to  this  portion  of  the  discus,  which 
.s  termed  the  corona  radiala  (Fig.  ,i,  cr).  Immediately  within  the 
corona  is  a  transparent  membrane,  the  zona  pellucida  (Fig  i,  zt^ 
aboutas  thickasoneof  the  cell  rows  of  the  corona  (0.02  too.024  mm  ' 
and  presenting  a  very  fine  radial  striation  which  has  been  held  to  be 
due  to  minute  pores  traversing  the  membrane  and  containing  delicate 
prolongations  of  the  cells  of  the  corona  radiata.  Within  Ihe  zona 
pellucida  IS  the  ovum  proper,  whose  cytoplasm  is  more  or  less  clearly 

St.  'It     T  '"  ""'"  ""'^   P^^'y  protoplasmic  portion 
(Fig.  II,  p)  and  an  inner  mass  (y)  which  contains  numerous  fine 
granules  of  fatty  and  albuminous  natures.    These  granules  represent 
th   food  yo  k  or  deutoplasm,  which  is  usually  much  more  abundant 
m  the  ova  of  other  mammals  and  forms  a  mass  of  relatively  enormous 
size  m  the  ova  of  birds  and  reptiles.    The  nucleus  („)'is  situated 
somewhat  excentncally  in  the  deutoplasmic  portion  of  the  ovum  and 
CO.  .tarns  a  single,  well-defined  nucleolus. 
I       A  follicle  with  the  structure  described  above  and  containing  a 
fully  grown  ovum  may  measure  anywhere  from  five  to  twelve  milli- 
meters in  diameter,  and  is  said  to  be  "mature,"  having  reached  its 
I  m  development  and  being  ready  to  burst  and  set  free  the  ovum 
This  however  is  not  ye*  mature;  it  is  not  ready  for  fertilization,  but 
must  first  undergo  certain  changes  similar  to  those  through  which 
the  spermatocyte  passes,  the  so-called  ovum  at  this  stage  being  more 
properly  a  primary  oocyte.    But  before  describing  the  phenomena  of 
maturation  of  the  ovum  it  will  be  well  to  consider  the  extrusion  of 
the  ovum  and  the  changes  which  the  follicle  subsequently  undergoes 
Ovulation  and  the  Corpus  Luteum.-As  a'rule,  but  a  sfng" 
follicle  near  maturity  is  found  in  either  the  one  or  the  other  ovary 
I  at  any  given  tunc.     In  the  early  stages  of  its  development  a  follicle 
IS  situated  somewhat  deeply  in  the  stroma  of  the  ovary,  but  during 
«s  growth  it  approaches  the  surface  and  eventually  forms  a  marked 


»»  OVULATION  AND  THE  CORPUS   lUTlUM 

prominence,  only  an  exceedingly  thin  membrane  separatinR  the 
cavity  of  the  follicle  from  the  abdominal  cavity.  Thi.  thin  mem- 
brane  finally  ruptures,  and  the  liquor  folliculi,  which  is  apparently 
under  some  pressure  while  contained  within  the  follicle,  rushes  out 
through  the  rupture,  carrying  with  it  the  ovum  surrounded  by  some 
of  the  cells  of  the  discus  proligerus. 

The  immediate  cause  of  the  bursting  of  the  follicle  is  not  yet 
clearly  understood.  It  has  been  suggested  that  a  gradual  increase 
of  the  liquor  folliculi  under  pressure  must  in  itself  finally  lead  to  a 
rupture,  and  it  has  also  been  pointed  out  that  just  before  the  matura 
hon  of  the  follicle  the  theca  interna  undergoes  an  exceedingly  rapid 
development  and  vascularization  which  may  play  an  important  part 
in  the  phenomenon. 

Normally  the  ovum  when  expelled  from  its  follicle  is  received  af 
once  mto  the  Fallopian  tube,  and  so  makes  its  way  to  the  uterus,  in 

whose  cavity  it  undergoes  its  de 
velopment.     Occasionally,  how 
ever,  this  normal  course  may  bt 
interfered  with,  the  ovum  coming 
to  rest  in  the  tube  and  there 
undergoing  its  development  and 
producing  a   tubal   pregnancy; 
or,  again,  the  ovum  may  not  find 
its  way  into  the  Fallopian  tube, 
but  may  fall  from  the  foUicli- 
into     the     abdominal      cavity, 
where,  if  it  has  been  fertilized, 
it    will    undergo   development, 
,    ^     „  ,  producing  an  abdominal  preg- 

nancy; and  finally,  and  still  more  rarely,  the  ovum  may  not  be 
expelled  when  the  Graafian  follicle  ruptures  and  yet  may  be 
fertilized  and  undergo  its  development  within  the  follicle,  bringing 
abou    what  IS  termed  .  .1  ovarian  pregnancy.    All  these  varietie' 

t  nT"/ir  ^'rr^  "'■  °^  '°""^'  ^-"edingly  serious,  sine, 
m  none  of  them  is  the  fetus  viable. 


FlO.    13— OVAHY  OP  A  WOUAN  NlNE- 

IKEN  Yeaiis  or  Aox,  Eight  Days  after 
Menstruation. 
d,  Blood-clot;  /  Graaffian  follicle;  Ih 
theca —(/To/Zwomi.) 


OVniAnON  AND  THE  COBPUS   lOTltJll 


»3 


With  the  setting  free  of  the  ovum  the  usefulness  of  the  Graafian 
.olhcle  »  at  an  end,  and  it  begins  at  once  to  undergo  retrogressive 
changes  which  result  primarily  in  the  formation  of  a  structure 
known  as  the  corpus  luleum  (Fig.  12).    On  the  rupture  of  the  follicle 


F.<=.  i3.-SEC„o»  THKOUOH  THE  Co.Py,  Lute™  0.  a  Rabbt,  Seventy  Hon« 

a  considerable  portion  of  the  stratum  granulosum  remains  in  place, 
and  usually  there  is  an  effusion  of  a  greater  or  less  amount  of  blood 
from  the  vessels  of  the  theca  interna  into  the  follicular  cavity  The 
si)l.t  m  the  wall  of  the  f^'-icle  through  which  the  ovum  escaped  .oon 
doses  over  and  the  cavity  becomes  filled  with  cells  separated  "into 
p-oups  by  trabecular  of  connective  tissue  containing  blood-vessels 
^J'  ig-  13).    These  cells  contain  a  considerable  amount  of  a  peculiar 


»4 


OVULATION  AND  THl  COBPnS   LVnvU 


yellow  pii^nt  known  as  lutein,  the  color  imparted  to  the  follicle 
by  this  substance  having  suggested  the  name  corpus  luteum  which 
IS  now  applied  to  it. 

In  later  stages  there  is  a  gradual  increase  in  the  amount  of  con- 
nective tissue  present  and  a  corresponding  diminution  of  the  lutein 
cells,  the  corpus  luteum  gradually  losing  its  yellow  color  and  be- 
coming converted  into  a  whitish,  fibrous,  scar-like  body,  the  corpus 
albicans,  which  mav  eventually  almost  completely  disappear     These 
various  changes  occur  in  every  ruptured  follicle,  whether  or  not  the 
ovum  which  was  contained  in  it  be  fertilized.    But  the  rapidity 
with  which  the  various  stages  of  retrogression  ensue  differs  greatly 
accordmg  to  whether  pregnancy  occurs  or  not,  and  it  is  customary 
to  distmguish  th-  corpora  lutea  which  are  associated  with  pregnancy 
^s  corpora  lutea  vera  from  those  whose  ova  fail  to  be  fertilized  and 
which  form  corpora  lutea  spuria.    In  the  latter  the  retrogression  of 
the  follicle  is  completed  usuall,  in  about  five  or  six  weeks,  while  the 
corpora  vera  persist  throughout  the  entire  duration  uf  the  pre^wncv 
and  complete  their  retrogression  after  the  birth  of  the  child 

Two  very  different  views  are  held  as  to  the  origin  of  the  lutein 
cells.  Accordmg  to  one,  which  may  be  termed  von  Baer's  view 
the  cells  of  the  stratum  granulosum  remaining  in  the  follicle  rapidly 
undergo  degeneration  and  completely  disappear,  and  the  lutein  cells 
and  connective-tissue  trabecute  are  formed  emirely  from  the  cells  of 
tJie  theca  mterna,  which  increase  rapidly  both  in  size  and  number 
The  other  view  was  first  advanced  by  Bischoff  and  may  be  known 
by  his  name.  It  is  to  the  effect  that  the  granulosa  cells  do  not  dis- 
integrate, but,  on  the  contrary,  increase  rapidly  in  number  and  be- 
come converted  into  the  lutein  cells,  only  the  connective  tissue  and 
the  blood-vessels  bemg  derived  from  the  theca  interna 

Which  of  these  two  views  is  correct  is  at  present  uncertain. 
The  majonty  of  those  who  have  within  recent  years  studied  the 
formation  of  the  human  corpus  luteum  have  expressed  themselves 
in  favor  of  von  Baer's  theory.  Sobotta  has,  however,  made  a 
thorough  study  of  the  phenomena  in  a  perfect  series  of  mice  ovaries 
and  has  demonstrated  that  in  that  form  the  lutein  cells  are  derived 


OVULATION  AMD  TBt  COKPUS  LtmSCII 


»s 


from  the  granulosa  cell*.  It  would  be  strange  if  the  lutein  cells  had 
a  different  origin  in  two  different  mammals,  and  the  observations  on 
mice  are  so  thorough  that  one  is  tempted  to  regard  different  results 
as  being  due  lo  imperfections  in  the  series  of  ovaries  studied, 
important  steps  in  the  development  of  the  corpora  lutca  being  thus 
overlooked.  This  temptation  is,  moreover,  greatly  increased  by  the 
fact  that  Sobotta's  observations  have  been  confirmed  in  the  cases  of 
several  other  animals,  such,  for  insUnce,  as  the  rabbit  ^Sobotta, 
Honor*,  Cohn),  certain  bats  (van  der  Stricht),  the  si.eep  (Marshall), 
the  marsupial  dasyurus  (Sandes),  the  spermophile  (Viilker),  and 
the  guinea-pig  (Sobotta).  The  weight  of  evidence  is  at  the  present 
time  strongly  in  favor  of  Bischoff's  view,  but  until  the  adverse 
results  obtained  by  Clarke  and  others  from  the  study  of  the  human 
corpus  luteum  and  those  obtained  by  Jankowski  fiom  the  pig  have 
been  shown  to  be  incorrect,  the  question  as  to  the  invariable  deriva- 
tion of  the  lutein  cells  from  the  stratum  granulosum  must  be  left 
open.  Since  it  is  held  that  both  the  granulosa  md  tlicca  cells  are 
derivatives  of  the  embryonic  ovarial  epithelium  the  essential  differ- 
ences between  the  two  origins  that  have  been  ascribed  to  the  lutein 
cells  may  not  be  so  great  as  has  been  supposed.  Indeed,  it  is  possible 
that  both  the  follicular  and  thecal  cells  may  in  some  cases  con- 
tribute to  the  formation  of  the  corpus  luteum. 

The  persistence  of  the  corpus  luteum  throughout  the  entire 
period  of  pregnancy  and  its  disappearance  within  a  few  weeks  if 
pregnancy  does  not  supervene,  have  suggt^.cd  the  probability  of  its 
being  related  to  the  changes  that  take  place  in  the  uterus  in  con- 
nection with  the  implantation  of  the  ovum  in  its  wall.  Experimental 
removal  of  the  corpus  luteum  in  rabbits  either  before  or  shortly 
after  the  implantation  of  the  ovum  produces  a  failure  of  pregnancy 
(Fraenkel),  and  similar  results  have  been  obtained  in  mice  and 
bitches  (Marshall  and  Jolly).  It  has  accordingly  been  held  Jhat 
the  corpus  JvUeumi^s  an  pr^ano^  secretion  directly  con- 

cernedliTthe  production  and  maintenance  af  the  ^modifications  of 
the  uterus  TOcesarj;^  for  the  implantation  and  further  development 
of  the  ovum. 


'6  THE  RELATION   OF   OVDIAHON  TO   MENST^HAHON 

peculiarities  and  by  climate  and  other  factors  anrf^f    -f  u 

«.r„r — s -- -  ssr  itrs"  E 


THE   RELATION   OF   OVUIATION   TO   MENSTRUATION  27 

real  significance  of  the  menstrual  cycle,  one  would  expect  to  find 
ovulation  occurring  at  a  morf  c;  los"!  definite  portion  of  the  cycle, 
at  such  a  time  that  the  ovu  u,  if  fertiliaci  would  be  able  to  make 
use  of  the  premenstrual  pre;  irrtion  for  ii ,  reception. 

Attempts  to  determine  the  ^"laticn  of  ovulation  to  menstruation 
have  been  made  by  estimating  the  age  ot  the  corpora  lutea  occurring 
in  ovaries  removed  in  the  course  of  operation  from  patients,  the  date 
of  whose  last  menstruation  was  known.  The  results  obtained  by 
this  method  have,  however,  proved  somewhat  discordant.  Thus, 
Fraenkel  records  out  of  eighty-five  cases  ten  in  which  the  operation 
was  performed  immediately  before  or  after  menstruation,  and  in 
none  of  these  was  any  corpus  luteum  present;  further,  in  twenty 
cases  a  newly  formed  corpus  luteum  was  found  and  in  these  cases 
the  last  menstruation  had  occurred  on  the  average  nineteen  (13-27) 
days  previously.  Villemin,  too,  reached  a  similar  result,  concluding 
that  ovulation  took  place  about  fifteen  days  after  menstruation. 
On  the  other  hand,  Leopold  and  Ravano  found  {hat  in  ninety-five 
cases  ovulation  coincided  with  menstruation  in  fifty-nine,  while  in 
the  remaining  thirty-six  it  occurred  during  other  stages  of  the  cycle. 

If  any  conclusion  may  be  drawn  from  these  contradictory  results 
it  would  seem  to  be  that  in  the  human  species  ovulation  may  take 
place  at  any  stage  of  the  menstrual  cycle.  Indeed,  it  may  also  be 
said  that  ovulation  may  take  place  independently  of  the  menstrual 
cycle,  since  cases  are  on  record  of  pregnancy  having  occurred  in 
girls  who  had  not  yet  menstruated.  In  other  words,  it  seems 
probable  that  ovulation  does  not  depend  upon  the  condition  of  the 
uterine  mucous  membrane,  but  upon  some  other  factor  as  yet 
undetermined. 

The  conditions  in  lower  animals  seem  also  to  point  in  this  direction 
In  these  ovulation  is,  as  a  rule,  associated  with  a  certain  condition  known 
as  (tstrus  or  "heat,"  this  being  preceded  by  certain  phenomena  con- 
stituting what  IS  termed  the  proastrum  and  corresponding  essentially  to 
menstruation.  In  several  forms,  such  as  the  dog  and  the  pig,  ovulation 
appears  to  occur  regularly  in  association  with  "heat,"  but  in  otiiers,  such 
as  the  cat,  the  mouse  and  probably  the  rabbit,  it  occurs  at  tiiis  time  only 
if  copulation   also  occurs.     Furthermore,  it  has  been  observed  that 


28 


IHE   MATDBATION   OF   THE   OVUM 


The  Maturation  of  the  Ovum     p.* 


THE    MATUXATION   OF   THE   OVUM 


29 


human  ova,  and,  indeed,  among  mammals  only  with  any  approach 
to  completeness  in  comparatively  few  forms  (rat,  mouse,  guinea- 
pig,  bat  and  cat);  but  they  have  been  observed  in  so  many  other 
forms,  both  vertebrate  and  invertebrate,  and  present  in  all  cases  so 


Fio. 


15- 


-DlAOKAlI  ILLOSTRATINO  THE    REDUCTION  Or"^IHE   ChuOMOSOMES  DCRING 

THE  Maturation  of  the  Ovum. 

0,  Ovum;  «',  oocyte  of  the  first  generation;  x',  oflcyte  of  the  second  generation; 

p,  polar  globule. 

much  uniformity  in  their  general  features,  that  there  can  be  little 
question  as  to  their  occurrence  in  the  human  ovum. 

In  typical  cases  the  ovum  (the  primary  oocyte)  undergoes  a 
division  in  the  prophases  of  which  the  chromatin  aggregates  fo  form 
half  as  many  tetrads  as  there  are  chromosomes  in  the  somatic  cells 


30 


THE  MArUBAHON  OF  IHE  OVUM 


stic^rci^'irfw^iLr  s.;r  ?r  --'  ^^^^^^  ^-- 

ary  oocytes)  are  not,  howeler  „  Tt  ""  '''o  «"« (second- 
almost  as  large  as  th^  oririn^l  n  '  "^'=  ""''  "^  "^^m  is 
called  an  ovu«  («J)  wEl  o^r'?'  ""'^'^  ""'*  ^°°*'""*«  '°  •>« 
A-fer  globule  (p)     AMnnVn             ''  """^  '°^"  *»'l  «  t««ed  a 

each  of  the  two  cdls  w'hS  '    f.  ^^.  ^"'^  "  ''"^le  chromosome  to 

half  the  n^b  r     rS"f„i^^^ 

The  second  diWs  on  lik^thT?-  =^^«'=™«<=  ^^  Ae  species. 

relatively  vej  w'  and  .n    . v  '  •"'  "°'''"^''  °"'  °^  *«  <=^"«  ^eing 

other  is  Sid  Ith^seconZr^  ^Kr""^  °^'""'  '"^'^  *»•' 
polar  giooule  divides  duZj^"  ^'°^"''-    ^"""^"t^^  ^^e  first 

species  ^  °^  chromosomes  characteristic  ior  the 


Oocyte/ 


o 

/  \ 


o 


/ 


Oocyte  II        Q 


/ 


Ovu 


,mO 


o 


P         O 


Spermato- 
cyte I 


Spermato- 
cyte II 


9^     O  O     O  Osp.™..,.. 


Pohr  tlobulei 


THE   FESTIUZATION  OP   THE   OVDU 


31 


during  the  process  of  reduction  of  the  chromosomes  only  to  undergo 
degeneration.  In  other  words,  three  out  of  every  four  potential 
ova  sacrifice  themselves  in  order  that  the  fourth  may  have  the  bulk, 
I  that  is  to  say,  the  amount  of  nutritive  material  and  cytoplasm  neces- 
sary for  efficient  development. 

The  Fertilization  of  the  Ovum.— It  is  perfectly  clear  that  the 
reduction  of  the  chromosomes  in  the  germ  cells  cannot  very  long  be 
repeated  in  successive  generations  unless  a  restoration  of  the  original 
number  takes  place  occasionally,  and,  as  a  matter  of  fact,  such  a 
restoration  occurs  at  the  very  begmning  of  the  development  of  each 
individual,  being  brought  about  by  the  union  of  a  spermatozoon 
with  an  ovum.  This  union  constitutes  what  is  known  as  the 
fertilization  of  the  ovum. 

The  fertilization  of  the  human  ovum  has  not  yet  been  observed, 
but  the  phenomenon  has  been  repeatedly  studied  in  lower  forms, 
and  a  thorough  study  of  the  process  has  been  made  on  the  mouse  by 
Sobotta,  whose  observations  are  taken  as  a  basis  for  the  following 
account. 

The  maturation  of  the  ovum  is  quite  independent  of  fertilization, 
but  in  many  forms  the  penetration  of  the  spermatozoon  into  the 
ovum  takes  place  before  the  maturation  phenomena  are  completed. 
This  is  the  case  with  the  mouse.  A  spermatozoon  makes  its  way 
through  the  zona  pellucida  and  becomes  embedded  in  the  cytoplasm 
of  the  ovum  and  its  tail  is  quickly  absorbed  by  the  cytoplasm  while 
its  nucleus  and  probably  the  middle-piece  persist  as  distinct  stnic- 
tures.  As  soon  as  the  maturation  divisions  are  completed  the  nucleus 
of  the  ovum,  now  termed  the/einale  pronucleus  (Fig.  16,  ek),  migrates 
toward  the  center  of  the  ovum,  and  is  now  destitute  of  an  archo- 
plasm  sphere  and  centrosome,  these  structures  having  disappeared 
after  the  completion  of  the  maturation  divisions.  The  spermatozoon 
nucleus, which,  after  it  has  penetrated  the  ovum,  is  termed  the  male 
pronucleus  (spk),  may  lie  at  first  at  almost  any  point  in  the  peripheral 
part  of  the  cytoplasm,  and  it  now  begins  to  approa:;h  the  female 
pronucleus,  preceded  by  the  middle-piece,  which  becomes  an  archo- 
plasm  spher;  with  its  contf.  :ned  centrosome  and  is  surrounded  by 


32 


THE   FEBTIllZATION  OP   THE   OVUM 


center  of  the  ovum,  forming  what  is   termed   the   seem^miZ 

hav  been  mtroduced  with  the  spermatozoon  undergo  divisLr  and 
he  two  archoplasm  spheres  so  formed  migrate  to  opposite  is  o1 
the  segmentat,o„  nucleus,  an  amphiaster  forms  and  the  corpound 
nucleus  passes  through  the  various  prophases  of  ruitos's  W 
m  the  mouse,  the  m.,le  and  female  pronuclei  have  ea  rcontribmed' 

of  twenty-four  chromosomes,  the  number  characteristic  forthe 
species  bemg  thus  restored. 

two^^I  '^''""''/"^  "'^  spermatogenesis  it  was  shown  (p.  i6)  that 
two  classes  of  spermatozoa  were  formed,  those  of  oneclass  cl 
.mmg  the  equivalent  of  twelve  chromosomes,  while  tho  e  ofThe 
other  Cass  contained  only  ten.  A  similar  seplration  of  the  ov^m 
nto^o  classes  probably  does  not  occur,  the  accessory  chron.oZe" 
m  the  oocytes  dmding  just  as  do  the  ordinary  ones,  so  thTerch 
ovum  possesses  twelve  chromosomes.  When,^herefC  the  u^ot 
of  the  male  and  female  pronuclei  takes  place  in  fertilization  those 
ova  that  are  fertilized  by  a  spermatozoon  with  twelve  cCosom 
W.I1  possess  twenty.four  of  these  bodies,  while  in  those  in  whlhThe 
omlr  T  r  ^^•^°™P"^'^^<»  by  a  spermatozoon  with  ten  cWo 

rTh  'f  ^Ir  !.'"'''*"°  "'"  "•='="'•    "^"^^  ""»t«  °f  chromosomes 
m  the  fertilized  ovum  determines  the  number  in  the  somatic  cTl 

ctsses  oT  "^K  '^'  '''"°P^  ''""^  "  '''^'^  ''^°«  there  will  be  two 
dasses  of  embzyos,  one  in  which  the  somatic  cells  possess  twemy 
four  chromosomes  and  another  in  which  there  are  trenty-twl 

That  this  condition  occurs  in  the  human  species  is  at  present 
merely  a  conjecture  based  partly  on  what  occurs  during  speS" 
gen«is  and  partly  on  what  has  been  shown  to  occur  b  7^^^^ 
o    invertebrates   (insects).    In  these,  two  classes  of  s^erltozoa 

diffenng  m  the  number  of  chromosomes  in  their  somatic  cells 
devdop  from  the  fertilized  ova;  and  it  has  been  furthert^d  ttai 
m  these  forms  those  with  the  greater  number  of  chromosome; 


THE   FERTILIZATION  OF   THE   OVUM 


33 


Fig.  16.-S1X  Stages  m  the  Process  op  Fertii.tmtion  of  the  Ovum  or  .  m. 

globules;  spk,  male  pronucleus.— (5o»ij«a.)  '      '      °  "»  Po'ar 

3 


34 


THE   FXSTIUZATION  OF  THE  OVUM 


become  females  and  those  with  the  smaller  number  males.  If,  as 
seems  probable,  this  condition  also  obtains  in  the  human  spedes, 
itJa-Bvirifnt  that  thejex-of  th&  future  individual  is  determined  at 
the  fertilization  of  the  ovumjinikcoiTeiated^with  the.number  of 
chromosomes  present  in  the  ovum  at_that_stag§. 

It  seems  to  be  a  rule  that  but  one  spermatozoon  penetrates  the 
ovum.  Many,  of  course,  come  into  contact  with  it  and  endeavor  to 
penetrate  it,  but  so  soon  as  one  has  been  successful  in  its  endeavor 
no  further  penetration  of  others  occurs.  The  reasons  for  this  are 
in  most  cases  obscure;  experiments  on  the  ova  of  invertebrates  have 
shown  that  the  subjection  of  the  ova  to  abnormal  conditions  which 
impair  their  vitality  favors  the  penetration  of  more  than  a  single 
spermatozoon  (fohypermy^,  and,  indeed,  it  appears  that  in  some 
forms,  such  as  the  common  newt  (Diemyctylm),  polyspermy  is  the 
rule,  only  one  of  the  spermatozoa,  however,  which  have  penetrated 
uniting  with  the  female  pronucleus,  the  rest  being  absorbed  by  the 
cytoplasm  of  the  ovum. 

Fertilization  marks  the  beginning  of  development,  and  it  is 
therefore  important  that  something  should  be  known  as  to  where 
and  when  it  occurs.  It  seems  probable  that  in  the  human  species  the 
spermatozoa  usually  come  into  contact  with  the  ovum  and  fertilize 
it  in  the  upper  part  of  the  Fallopian  tubes,  and  the  occurrence  of 
extra-uterine  pregnancy  (see  p.  22)  seems  to  indicate  that  occasion- 
ally the  ovum  may  be  fertilized  even  before  it  has  been  received  into 
the  tube. 

It  is  evident,  then,  that  when  fertilization  is  accomplished  the 
spermatozoon  must  have  traveled  a  distance  of  about  twenty-four 
centimeters,  the  length  of  the  upper  part  of  the  vagina  being  taken 
to  be  about  5  cm.,  that  of  the  uterus  as  7  cm.,  and  that  ot  the  lube 
as  12  cm.  A  considerable  interval  of  time  is  required  for  the  com 
pletion  of  this  journey,  even  though  the  movement  of  the  spermat- 
ozoon be  tolerably  rapid.  The  observations  of  Henle  and  Hensen 
indicate  that  a  spermatozoon  may  progress  in  a  straight  line  at  about 
the  rate  of  from  1,2  to  2.7  mm.  per  minule,  while  Lott  finds  the  rat 
to  be  as  high  as  3.6  mm.    Assuming  the  rate  of  progress  to  be  about 


IHE  FMTILIZATION  Of  THE   OVDM  35 

JS  mm.  per  minute,  the  time  required  by  the  spermatozoen  to 
travel  from  the  upper  part  of  the  vagina  to  the  upper  part  of  a 
Fallopian  tube  will  be  about  one   .nd  a  half  hours  (Strassmann) 
This,  however,  assumes  that  ther<-  arc  no  obstacles  in  the  way  of  the 
rapid  progress  of  the  spermatozoon,  which  is  not  the  case,  since  in 
the  first  place,  the  irregularities  and  folds  of  the  lining  membrlne 
of  the  tube  render  the  path  of  the  spermatozoon  a  labyrinthme  one, 
and,  secondly,  the  action  of  the  cilia  of  the  epithelium  of  the  tube 
and  uterus  being  from  the  ostium  of  the  tube  toward  the  os  uteri  it 
will  greatly  retard  the  progress;  furthermore,  it  is  presumable  that 
the  rapidity  of  movement  of  the  spermatozoon  diminishes  after  a 
certam  mterval  of  time.    It  seems  probable,  therefore,  that  fertili- 
zation does  not  occur  for  some  hours  after  coition,  even  providing 
an  ovum  is  in  the  tube  awaiting  the  approach  of  the  spermatozoon 
But  this  condition  is  not  necessarily  present,  and  consequently 
the  question  of  the  duration  of  the  vitality  of  the  sperm  cell  becomes 
of  miportance.    Ahlfeld  has  found  that,  when  kept  at  a  proper 
temperature,  a  spermatozoon  will  retain  its  vitality  outside  the  body 
for  eight  days,  and  Duhrssen  reports  a  case  in  which  living  spermat- 
ozoa were  found  in  a  Fallopian  tube  removed  from  a  patient  who 
had  last  been  m  coitu  about  t„ree  and  a  half  weeks  previously 
As  regards  the  duration  of  the  vitality  of  the  ovum  less  accurate  data 
are  available.    Hyrtl  found  an  apparently  normal  ovum  in  the 
uterme  portion  of  the  left  tube  of  a  female  who  died  three  days  after 
the  occurrence  of  her  second  menstruation,  and  Issmer  estimates 
the  duration  of  the  capacity  for  fertilization  of  an  ovum  to  be  about 
sixteen  days. 

It  is  evident,  then,  that  even  when  the  exact  date  of  the  coitus 
which  led  to  the  fertilization  is  known,  the  actual  moment  of  the 
latter  process  can  only  be  approximated,  and  in  the  immense  ma- 
jority of  cases  it  is  necessary  to  rely  upon  the  date  of  the  last  men- 
struation for  an  estimation  of  the  probable  date  of  parturition 
And  by  this  method  the  possibilities  for  error  are  much  greater' 
siuce,  as  been  pointed  out,  ovulation  is  not  necessarily  associated 
with  menstruation.    The  duration  of  pregnancy  is  normally  ten 


il 


36 


UTESATDSE 


unar  or  about  mne  calendar  months  and  it  is  customary  to  estimate 
he  probable  date  of  parturition  as  nine  months  and  sev«n  days 
from  the  last  menstruation.    From  what  has  been  said,  it  is  clear 
that  any  such  estimation  can  be  depended  upon  only  as  an  approxi- 
mation, the  possible  variation  from  it  being  considerable. 

Superfetation.— The  occasional  occurrence  o'.  twin  fetuses  indiffprmt 
Stages  of  development  has  suggested  the  possibiUty  of  Kt  Uzado"  o 

f^°^^  f""  "  '^^  ■■"""  °'  »  ""^on  "fa"  appreciable  interval  of  tLe 
after  the  first  ovum  has  started  upon  its  development  Ther^tenSl 
^JZltZ"  'r  '"""""K.*'"  "any  of  the  cases  of  sup^sed  ^ 
/«/a|.«.  as  this  phenomenon  is  termed,  are  instances  of  the  simultan^, 

ertibzation  of  two  ova,  one  of  which,  for  some  cauL  concTme5  wiA 

At  th??i°^-"'"'l°"'  *"" '"'"  ^"^^"^  '°  '^'''^°f>  a"  rapidly  ^TeotJer 
At  the  same  time,  however,  even  although  the  phenomenon  may  b«  of 
rare  occurrence,  it  is  by  no  Wans  impossible,  for  occasionally  a  s,?ond 
Graafian  folhc  e,  either  in  thr  same  or  the  other  ovaiy,  may  Linear 
ZS;?'^  '^fS^v.T"  ''  ^^""'*<*  «~"  after  the  S  "nJ^  aSd  ifX 
m/mhf,  i"'  '^^  'f ""  ""^J^^  '""'■'""'l  Ganges  in  the  uteri^mucous 
membrane  have  not  proceeded  so  far  as  to  prevent  the  access  of  ?hp 
spermatozoon  to  the  ovum,  its  fertilization  and  devetpment  ^  ens^e 
The  changes,  however,  which  prevent  the  passage  of  the  sStozofin 
are  completed  early  in  development  and'^Uie  difference  bet?^™  the 
normjiUy  developed  embryo  and  that  due  to  superfetadon  wffl  brcom 
pamnvely  smaU  and  wiU  become  less  and  less  evident  ^  d^elop^ni 
proceeds,  provided  that  the  supply  of  nutrition  to  both  embryos  fa  e^d! 

LITERATURE. 
E.  Buiowiiz:  "IJntersuchungen  ttber  die  Struktur  der  Sp«mato«M, "  No    a 

K.  VON  Bajdelmin:  "Beilrtge  zur  Histologic  des  Hodens  und  zur  Spermatogcnese 
bdm  M^<^.-ArMv/ur  Ana,,  und  PkyM..  Ana,.  AM..  Supt^Tj 

J.  G.  Cla«:     Ursprung,  WachsUium  und  Ende  des  Corpus  luteum  nach  Bwbach 

^'  ?.S^!^U"""  '"  ""'""*  '"  Richtungslcorper  bei  Mus  musculus,"  Wies 
S.  Girmaz:  «u.b«  eta  bemcrkenswerte,  Strukturelement  (H«eRx:hTomosome)  ir 

d«- Sp«m.ogen«.  das  Mens-bcn,"  ^rrf  /  M.ir.  An^.,  txxix.  zo,. 
M.  F.  GnvER:  "Accessory  Chromosomes  in  Man,"  Biol.  BuU.,  Hx  igio  ' 
W.  H«„;     The  Sexual  S.a«>n  of  Mammals  .^  Ure  Relation  of  the  PrtxMtrum  to 


UTESATUSE 


37 


MoutrmUon,"  0«*/.  y««.  yi„„.  Sci.,  N.  S.,  xm,  .901  (coBttta.  very  full 
bibliognphy). 

O.  H««Twio:  "Vetglelch  An  Ei-  und  SaracnbUdung  bci  Ntnuloden,"  HrrtwA, 
mikrMk.  Anat.,  xxxvi,  1890. 

F.  HiTSCHlUNN  and  L.  Adim:  "Der  Bau  d«  Uttru»K:hleimhaut  d«  gMchl«rhl^ 

reiJni  Wnb«,  mit  bnondtrer  Bcrtlck.lchUgung  der  Mm.tnialion,"  Umals,chr. 

/tir  GAnrtsh.  mdGyH<uk.,xxxn,it)Olt. 
J.  J.wkowsh:  "Beitrag  lur  EnUtehung  des  Corpus  lutcum  dcr  SSugctiere  "  Arch  f 

mikr.  Anal.,  txiv,  1904. 
W.  B.  KniHAll:  "The  Maturation  of  the  Mouse  Egg,"  Biol.  BuUelin.  xn  i(^ 
H.  Lahs  and  J.  Doona:  "Nouvelles  recherches  jur  la  maturation  et  la  Krondktion 

de  I'oeuf  de  mammiRres,"  Arch,  dt  Biol.,  xxlli,  igo?. 
M.  VON  Lliraossilc:  "Untersuchungen  ttber  Spermatogenese,"  Archiv  fur  mikrosk 

Amu.,  u,  l8q8, 

G.  IJBOPOlD  and  A.  Rovano:  "Neuer  Beitrag  zur  Lehre  von  der  Menstruation  und 

OvuUtion,"  Areh./urCytiiuk.,  lxxxiii,  1007. 
W.  H.  Lonoliy:  "The  Maturation  of  the  Egg  and  Ovulation  in  the  Domestic  Cat  " 

Amir.  Journ.  Anai.fXn,  igit.  ' 

F.  a  A.  Mambail:  "The  (Estrus  Cycle  and  the  Formation  of  the  Corpus  luteum  in 

the  Sheep,"  P.jfoj.  Trans.,  Sir.  B,  cxcvi,  1904. 
F.  H.  A.  MA«iH'.LL:  "The  Development  of  the  Corpus  luteum:  a  Review  "  0,^ 

Joum.  Micros.  Sci.,N.S.,\Llx,igo6. 

F.  Mevb:  "  Ueber  Slruktur  und  Histogenese  der  SamenfMen  de>  MectKhweinchens," 

Archnfilr  mikrosk.  Anal.,  Liv,  1899. 

T.  H.  Mostooiimy:  "Differentiation  of  the  human  Cells  of  SertoU  "  Bioloi  BuU 
XXI,  igii.  *  ■' 

W.  Naoel;  "Das  menschUche  Ei,"  Archiv  far  mikrosk.  Anal.,  xxxi  1888 

G.  NressiNo:  "  Die  BetheUigung  der  Centralkorper  und  Sphere  am  Aufbau  des  Samen- 

fadens  bei  S»ugethieren,"  Archiv  fur  mikrosk.  Anal.,  xvrm,  i8g6. 
G.  RiTznrs:  "Die  Spermien  des  Menschen,"  Btolog.  Vntersuch.  xiv   1909 
W.RraASCHKD.:  "Ueber  die  Reifungs- und  Befruchtungs-processe  des  Mecrschwein- 

cheneres,"  Anat.  Hejle,  xxix,  1905. 
J.  Sobotta:  "Die  Befruchtung  und  Furchung  des  Eies  der  Maus,"  Archiv  far  mikrosk 

Anat.,XLV,  1895. 

J.  Sobotta.-  "  Ueber  die  Bildung  des  Corpus  luteum  bei  der  Maus,"  Archiv  far  mikrosk 
Anal.,  XLVII,  1897. 

J.  Sobotta:  "Ueber  die  BUdung  des  Corpus  lutcum  beim  Meerschweinchen." ^m« 
Hefte,  XXXII,  1906. 

J.  Sobotta  and  G.  Bukckhabd:  "Reifung  und  Befruchtung  der  Eier  des  weissen 
Ratte,"  Anat.  He/le,  XLii,  1910. 

P.  Stsassiunn:  "Beitrage  zur  Lehre  von  dw  Ovulation,  Menstruation  und  Concep- 
tion,   Archiv  far  Gymukii.,  in,  1896. 

1'.  Villeiiin:  "Le  corps  jaune  consid<r^  comme  glande  H  s&r^tion  interne  "  Paris 
190S.  '         "' 

W.  Waldevm:  "Eierstock  und  Ei,"  Leipzig,  1870. 


CHAPTER  n. 

THE  SEGMEIf  TATlOir  OF  THE  OVUM  AMD  THE  FORMATIOH 
OF  THE  OESM  LAYERS. 

Segmentation.— The  union  of  the  male  and  female  pronuclei 
has  already  been  described  as  being  accompanied  by  the  formation 
of  a  mitotic  spindle  which  produces  a  division  of  the  ovum  into  two 
cells.  This  first  division  is  succeeded  at  more  or  less  regular 
mtervals  by  others,  until  a  mass  of  cells  is  produced  in  which  a 
differentiation  eventually  appears.  These  divisions  of  the  ovum 
constitute  what  is  termed  its  segmentation. 

The  mammalian  ovum  has  behind  it  a  long  line  of  evolution 
and  even  at  early  stages  in  its  development  it  exhibits  peculiarities 
which  can  only  be  reasonably  explained  as  an  inheritance  of  past 
conditions.  One  of  the  most  potent  factors  in  modifying  the 
character  of  the  segmentation  of  the  ovum  is  the  amount  of  food 
yolk  which  it  contains,  and  it  seems  to  be  certain  that  the  immediate 
ancestors  of  the  mammalia  were  forms  whose  ova  contained  a  con- 
siderable amount  of  yolk,  many  of  the  peculiarities  resulting  from 
Its  presence  being  still  clearly  indicated  in  the  early  development  of 
the  almost  yolkless  mammalian  ovum.  To  give  some  idea  of  the 
peculiarities  which  result  from  the  presence  of  considerable  amounts 
of  yolk  It  will  be  well  to  compare  the  processes  of  segmentation  and 
differentiation  seen  in  ova  with  different  amounts  of  it. 

A  little  below  the  scale  of  the  vertebrates  proper  is  a  form, 
Amphioxus,  which  possesses  an  almost  yolkless  ovum,  presenting  a 
simple  process  of  development.  The  fertilized  ovum  of  Am^ioxus 
;n  Its  first  division  separates  into  two  similar  and  equal  cells,  and 
these  are  made  four  (Fig.  17,  A)  by  a  second  plane  of  division 
which  cuts  the  previous  one  at  right  angles.    A  third  plane  at 

38 


SEOMZNTATION  Of  THE   OVCIi 


39 


right  angles  to  both  the  preceding  ones  brings  about  an  eight-celled 
stage  (Fig.  17,  B),  and  further  divisions  result  in  the  formation 
of  a  large  number  of  cells  which  anange  themselves  in  the  form 
of  a  hollow  sphere  which  is  known  as  a  blaslula  (Fig.  17,  E). 

The  minute  amount  of  yolk  which  is  present  in  the  ovum  of 
Aml^ioxus  collects  at  an  early  stage  of  the  segmentation  at  one  pole 
of  the  ovum,  the  cells  containing  it  being  somewhat  larger  than  those 
of  the  other  pole  (Fig.  17,  B),  and  in  the  blastula  the  cells  of  one  pole 
arc  larger  and  more  richly  laden  with  yolk  than  those  of  the  other 
pole  (Fig.  17,  F).  If,  now,  the  segmenting  ovum  of  an  Amphibian 
be  examined,  it  will  be  found  that  a  very  much  greater  amount  of 


Fio.  17. — Staou  in  thk  Seoiontation  of  Ampkiaau. 

A,  Faiir<eUed  stage;  B,  ciEht-^cUed  stage;  C,  siiteen-celled  stage;  D,  early  blaslula;  JE> 

blastula;  F,  section  at  blastula.— (/fiKic***.) 


yolk  is  present  and,  as  in  Amphioxus,  it  is  located  especially  at  one 
pole  of  the  ovum.  The  first  three  planes  of  segmentation  have  the 
same  relative  positions  as  in  Amphioxus  (Fig.  17),  but  one  of  the 
tiers  of  cells  of  the  eight-celled  stage  is  very  r"uch  smaller  than  the 
other  (Fig.  18,  B).  In  the  subsequent  stages  of  segmentation  the 
small  cells  of  the  upper  pole  divide  more  rapidly  than  the  larger  ones 
of  the  lower  pole,  the  activity  of  the  'atter  seeming  to  be  retarded  by 
the  accumulation  of  the  yolk,  ar  resulting  blastula  (Fig.  18, 


40 


THE   SEGMENTATION   OF   THE   OVUM 


D)  shows  a  very  decided  difference  in  the  size  of  the  cells  of  the  two 
poles. 

In  the  ova  of  reptiles  and  birds  the  amount  of  yolk  stored  up  in 
the  ovum  is  very  much  greater  even  than  in  the  amphibia,  and  it  is 
aggregated  at  one  pole  of  the  ovum,  of  which  it  forms  the  principal 
mass,  the  yolkless  protoplasm  appearing  as  a  small  disk  upon  the 


c  D 

Fig.  s8.— Stages  in  tbi  Segmkntation  o»  AuMytoma.—iEydahynur.) 

surface  of  a  relatively  huge  mass  of  yolk.  The  inertia  of  this  mass  of 
nutritive  material  is  so  great  that  the  segmentation  is  confined  to  the 
small  yolkless  disk  of  protoplasm  and  aftects  consequently  only  a 
portion  of  the  entire  ovum.  To  distinguish  this  form  of  segmenta- 
tion from  that  which  affects  the  entire  ovum  it  is  termed  meroUasHc 
segmentation,  the  other  form  being  known  as  holoUaslic. 

In  the  ovum  of  a  turtle  or  a  bird  the  first  plane  of  segmentation 
crosses  the  protoplasmic  disk,  dividing  it  into  two  practically  equal 


THE   SEGHENTATION  OF   THE   OVUM 


41 


halves,  and  the  second  plane  forms  at  approximately  right  angles 
to  the  first  one,  dividing  the  disk  into  four  quadrants  (Fig.  19,  A). 
The  third  division,  like  the  two  which  precede  it,  is  radial  in  position, 
while  the  fourth  is  circular  and  cuts  off  the  inner  ends  of  the  six 
cells  previously  formed  (Fig.  19,  C).  The  disk  now  consists  of 
six  central  smaller  cells  surrounded  by  six  larger  peripheral  ones. 


Fig.  19. — FouK    Stages  in  the    Segmentation    of   the  Blastodekh  of  the 
Chicc.— (Cojfe.) 

Beyond  this  period  no  regularity  can  be  discerned  in  the  appearance 
of  the  segmentation  planes;  but  radial  and  circular  divisions  con- 
tinuing to  form,  the  disk  becomes  divided  into  a  large  number  of 
cells,  those  at  the  center  being  much  smaller  than  those  at  the  per- 
iphery. In  the  meantime,  however,  the  smaller  central  cells  have 
begun  to  divide  in  planes  parallel  to  the  surface  of  the  disk,  which, 
from  being  a  simple  plate  of  cells,  thus  becomes  a  discoidal  cell- 


4a 


THE   SEGIIENXATION   OF  THE   OVOM 


in  »Sr^*»  *e.  Wntation  of  the  disk  it  has  inc«ased  materially 

m  sue,  extendmg  further  and  further  over  the  surface  of  the^li 

mo  the  substance  of  which  some  of  the  lower  cells  of  the  discoSai 

en-mass  have  penetrated.    A  comparison  of  the  diagram  S 

th   We  7r  °' '  '^If '  'I  ^'°"'  «'■'  ^'''K^  °f  deveient  witt 
the  figure  of  the  amphibian  blastula  (Fig.  i8,  D)  will  indicate  the 

simdaruy  between  thetwo.  the  largeyolk-massy)  of  the  reptM 
the  scattered  cells  which  it  contains  corresponding  to  the  lo'wer  ^t 


N,  Blastoderm;  Y,  yolk-mass. 

cells  of  the  amphibian  blastula,  the  central  cavity  of  which  is  prac 
ticaUy  suppressed  in  the  reptile.  Beyond  this  stage,  however  X 
s.m,  anty  becomes  more  obscured.  The  peripheral  c  lis  oXdi 
contmue  to  extend  over  the  surface  of  the  yolk  and  final  ycompleS 
ly  enclose  U,  forming  an  enveloping  layer  which  is  compIetedTt  the 

zKet^:r ^  *^  '"-°^^^'  -"— '  -  -^^  ^^  -% 

in  tl^f  ""^  •"  *'  '°"»'»^''^'*  it  wUl  be  found  that  the  ovum 
m  the  great  majonty  as  almost  or  quite  as  destitute  of  food  yolk  as  is 

*  The  segmentation  of  the  human  Oram  has  nnt  »-t  i i,         ,      ■ 


THE   SEGMENTATION  OF   THE   OVUM 


43 


the  ovum  of  Amphioxus,  with  the  result  that  the  segmentation  is  of 
the  total  or  holoblastic  type.  It  does  not,  however,  proceed  with 
that  regularity  which  marks  the  segmentation  of  Amphioxus  or  an 
amphibian,  but  while  at  first  it  divides  into  two  slightly  unequal 
cells  (Fig.  2i),  thereafter  the  divisions  become  irregular,  three-celled. 


Fio.  ai. — FouK  Stages  in  the  Segmentation  of  the  Ovuk  or  a  Mouse 
X ,  Polar  globule.— (So6o«o.) 

four-celled,  five-celled,  and  six-celled  stages  having  been  observed 
in  various  instances.  Nor  is  .he  result  of  the  final  segmentation  a 
hollow  vesicle  or  blastula,  but  a  solid  mass  of  cells,  termed  a  morula, 
is  formed.  This  structure  is  not,  however,  comparable  to  the  blas- 
tula of  the  lower  forms,  but  corresponds  to  a  stage  of  reptilian  devel- 
opment a  little  later  than  that  shown  in  Fig.  20,  since,  as  will  be 
shown  directly,  the  cells  corresponding  to  the  blastoderm  and  the 


44 


THE  SEGlCEmATION  OF   THE   OVtJM 


enveloping  layer  are  already  present.    There  is,  then,  no  Wastula 
stage  in  the  mammalian  development. 

Differentiation  now  begins  by  the  peripheral  cells  of  the  morula 
becommg  less  spherical  in  shape  and  later  forming  a  layer  of  flat- 
tened cells,  the  enveloping  layer,  surrounding  the  more  spherical 
central  cells  (Fig.  22.  A).  In  the  latter  vacuoles  now  make  their 
appearance,  especially  in  those  cells  which  are  nearest  what  may  be 
regarded  as  the  lower  pole  of  the  ovum  (Fig.  22,  C),  and  these 
vacuoles,  gradually  increasing  in  size,  eventually  become  confluent, 
the  condition  represented  in  Fig.  22,  D,  being  produced.  At  this 
stage  the  ovum  consists  of  an  enveloping  layer,  enclosing  a  cavity 
which  is  equivalent  to  thie  yolk-mass  of  the  reptilian  ovum,  the 
vacuoli;,;  ;,.;i  of  the  inner  cells  of  the  morula  representing  a  belated 
formation  of  yolk.  On  the  inner  surface  of  the  enveloping  layer, 
at  what  may  be  termed  the  upper  pole  of  the  ovum,  is  a  mass  of  cells 
projecting  into  the  yolk-cavity  and  forming  what  is  known  as  the 
inner  cell-mass,  a  structure  comparable  to  the  blastoderm  of  the 
reptile.  In  one  respect,  however,  a  difference  obtains,  the  inner 
cell-mass  being  completely  enclosed  within  the  enveloping  cells, 
which  is  not  the  case  with  the  blastoderm  of  the  reptile.  ~>at 
portion  of  the  enveloping  layer  which  covers  the  cell-mass  has  been 
termed  Rauber's  covering  layer,  and  probably  owes  its  existence  to  the 
precocity  of  the  formation  of  the  enveloping  layer. 

It  is  clear,  then,  that  an  explanation  of  the  early  stages  of 
development  of  the  mammalian  ovum  is  to  be  obtained  by  a  com- 
parison, not  with  a  yolkless  ovum  such  as  that  of  Amphioxus,  but 
with  an  ovum  richly  laden  with 'yolk,  such  as  the  meroblasti.- 
ovum  of  a  reptile  or  bu-d.  In  these  forms  the  nutrition  necessan 
for  the  growth  of  the  embryo  and  for  the  complicated  processes 
of  development  is  provided  for  by  the  storing  up  of  a  quantity 
of  yolk  in  the  ovum,  the  embryo  being  thus  independent  of  externa! 
sources  for  food.  The  same  is  true  also  of  the  lowest  mammalia, 
the  Monotremes,  which  are  egg-laying  forms,  producing  ova 
resembling  greatly  those  of  a  reptile.  When,  however,  in  th, 
higher  mammals  the  nutrition  of  the  embryo  became  provided 


THE   SEGMENTATIOK   OF   THE    OVUH 


45 


C  D 

Fio.  22. — Later  Stages  m  thi  Segmentation  of  the  Ovum  oi  a  Bat. 
i4,  C,  and  D  are  sections,  B  a  surface  view. — {Van  Baudm,) 


46 


I-WIN  DEVnoPlttNT  AND  DOOTU  MONSTERS 


S  th^'™  '^"'*  "^  "^^  '"'^'y"  '°  'J^*  -«"«  Of  the  Uterus 

of  the  parent  so  that  it  could  be  nourished  directly  by  the  mZT 
the  stonng  up  of  yolk  in  the  ovun.  was  unnecessa/and  if  ^S 
hotoblasfcov^,  although  many  peculiarities  Zpendem  r^! 
onguu.1  meroblasfc  condition  persisted  in  its  devebpm^t. 

and  triplets  and 'even  quSSS^.?^ '?,''>' "°  T^  i^f"*!'-"'. 
occurrence  of  twias  may  K^C^i^^"'^^  "*  developed.^  The 
ripening  and  ferSL^f  ^^  ^a^S  W  1?' "^^^r.^ 
ovanes,ortothes.;parationofasinBlefrr«M,!.H  ?°'  °'  '"""  '^^ 

ent  parts  during  the  early  steees  of  ri^v^^  """^1?'°  ^  i°depend- 
produced  by  t4  litter  oioc^h.l  w  T"'5'-  ^'•»'  '™»  «»^  be 
mentadonu^poT^e'^IoK^To^w^L^^^^^^^  •'^  «^P«ri- 

Amphioxus oVum  in  thit  stoBrofH«,rn,r^' ',  '^  °.'  ^"^  *^°  ""*  °f  an 
completing  its  developmerS,d  n^^f^"""''  "*  ™'*anicaUy  separated, 
normal  si^.     "'"'''^P^'''  and  produang  an  embiyo  of  about  haif  the 

occ^e  oilZl  ^nstt  ^i^^^,  '/  ^-^-T'>'=  occasional 
into  two  parts  of  roriS":i^',:^i^„^  ^Z  r 'T'ff '  ^P"*""" 
and  probably  also  the  stage  of  dCT?onmen7^;  if-  ?^'"  °^  *=  separation, 
the  amount  of  fusion  olX  ^wS"'??^^  *'  "^^^  "  O'^"'"'  determining 
gradations  of  sepSn  occT  ram  ^"1  ,^°-"'^'y«"«  ''•«  -"onster.  All 
in  such  cases  as  the  S  °a  ne^t^s  tol^™!  •  ""^  "u  l«P"a«on.  as  seen 
are  united  diroughout  the  enSn  ^i^^^""-  "^^  *'  '*°  individuals 
may  also  affect  on?y  a  i^S^Tof  th?,l  *"  '^/'•.  ^'''  ^^Paration 
double-faced  or  duuble-^eSed  mnn  ,        '^°'  P^dudng,  for  instance, 

parasitic  monsters;  ant  SyitZ^aff..7  ^"^""^  '""^  "'  ^'-"^^d 
to  form  a  spedal  orgaiTrodudr^an  1  """'f  ^"'"P  °^ ''"^  destined 

numerary  di'gits  or  acTs'soo^  spS  *''  P"^'  '"^  "^  '"P*'" 

twofu^di^ridtu'IllllJh^a^r' "•h""''!''-  ""°"^'-  '^'^  0-  of  *e 
it  being,  as  it  were   d,?Lki^.  „tr''-''°°  °' '^  r"io"s  organs  reversed, 

observed  in  single  individuals  ^d  a  oSle  ^1  f'^^^'^'^y  been 
regards  them  as  one  of  a  nair  nf  L.v'^  f       ?  explanation  of  such  cases 

eibryo.  die  oti.erTd^vidJ'^^h^kTS^'d'^'dlt 
undergone  degeneration  or,  if  tS^n^ati^„  if^P  "?•*  *''^f  ""^"g 
being  induded  witiun  die  body  of  S.T?°"  7*1,  *°  .'"?'"P'^'=  one, 
Anodier  explanation  of  sihisTI  °!.^    apparently  smgle  individ-al. 

the  basis  of  what  has  beTobS'in^e,^'^"-"'^^^"'  ^^^"^^  "n 
spedes  of  snails  situs  inveS^st'T^, '"."^T'?"  "^"''•'rates.     In  some 

that  the  inversion  may  b^^ ed^^IcTK^ S^^^""  ^.^^ 


F0UUT10M  OF  THE  OEKH  LAYERS 


47 


earliest  segmenUtion  stages.  The  conclusion  is  thereby  indicated  that 
its  primary  cause  may  reside  in  an  inversion  of  the  polarity  of  the  ovum, 
evidence  being  forthcoming  in  favor  of  the  view  that  even  in  the  ovum 
of  these  and  other  forms  there  is  probably  a  distinct  polar  differentiation. 
How  far  this  view  may  be  applicable  to  the  mammalian  ovum  is  uncertain, 
but  if  it  be  applicable  it  explains  the  phenomenon  of  inversion  without 
complicating  it  with  the  question  of  twin-formation. 

The  Formation  of  the  Germ  Layers.— During  the  stages 
which  have  been  described  as  belonging  to  the  segmentation  period 
of  development  there  has  been  but  little  differentiation  of  the  cells. 
In  Amfhioxus  and  the  amphibians  the  cells  at  one  pole  of  the  blastula 
are  larger  and  more  yolk-laden  than  those  at  the  other  pole,  and  in 
the  mammals  an  inner  cell-mass  can  be  distinguished  from  the 
enveloping  cells,  this  latter  differentiation  having  betn  anticipated  in 
the  reptiles  and  being  a  differentiation  of  a  portion  of  the  ovtim  from 
which  alone  the  embryo  will  develop  from  a  portion  which  will  give 


A  B 

Fig.  33. — ^Two  Stages  in  the  Gastrulation  of  Ampkioxus. — (Aforgan  and  Hatm.) 

rise  to  accessory  structures.  In  later  stages  a  differentiation  of  the 
inner  cell-mass  occurs,  resulting  first  of  all  in  the  formation  of  a  two- 
layered  or  diploblastic  and  later  of  a  three-layered  or  triploblastic 
stage. 

Just  as  the  segmentation  has  been  shown  to  be  profoundly 
modified  by  the  amount  of  yolk  present  in  the  ovum  and  by  its  sec- 
ondary reduction,  so,  too,  the  formation  of  the  three  primitive  layers 


48 


FOailATION  OF  THE  OEBM  tAYKU 


is  much  modified  by  the  same  cause,  and  to  get  a  clear  understanding 
of  the  formation  of  the  triploblastic  condition  of  the  mammal  it  will 
be  necessary  to  describe  briefly  its  development  in  lower  forms. 

In  A  mphioxus  the  diploblastic  condition  results  from  the  flattening 
of  the  large-celled  pole  of  the  '  lastula  (Fig.  33,  A),  and  finally  from 
the  invagination  of  this  portion  of  the  vesicle  within  the  other  portion 
(Fig.  23,  B) .  The  original  single- walled  blastula  in  this  way  becomes 
converted  into  a  double-walled  sac  termed  a  gaslnda,  the  outer  layer 
of  which  is  known  as  the  ectoderm  or  epiblast  and  the  inner  layer  as 
the  mdoderm  or  hypoblast.  The  cavity  bounded  by  the  endoderm  is 
the  primitive  gut  or  archentiron,  and  the  opening  by  which  this 
communicates  with  the  exterior  is  the  blastopore.  This  last  structure 
is  at  first  a  very  wide  opening,  but  as  development  procee'ln  il 

becomes  smaller,  and  finally  is  a 
relatively  small  opening  situated  at 
the  posterior  extremity  of  what 
will  be  the  dorsal  surface  of  the 
embryo. 

As  the  oval  embryo  continues 
to  elongate  in  its  later  development 
the  third  layer  or  mesoderm  makes 
its  appearance.  It  arises  as  a 
lateral  fold  {mp)  of  the  dorsal  sur- 
face of  the  endoderm  (en)  on  each 
side  of  the  middle  line  as  indicated 
in  the  transverse  section  shown  in 
Fig.  24.  This  fold  eventually  be- 
comes completely  constricted  off 
from  the  endoderm  and  forms  a 
hollow  plate  occupying  the  space  between  the  ectoderm  and  endo- 
derm, the  cavity  which  it  contains  being  the  body-cavity  or  ccelom. 

In  the  amphibia,  where  the  amount  of  yolk  is  very  much  greater 
than  in  Amphioxus,  the  gastrulation  becomes  considerably  modified. 
On  the  line  where  the  large-  and  small-celled  portions  of  the  blastula 
become  continuous  a  crescentic  groove  appears  and,  deepening, 


Fio.  J4. — TuANSVEitsi  Section  of 
Amphioxus  Eubkyo  with  Five  Mxso- 
DEKific  Pouches. 

Ck,  Notochord;  d,  digestive  cavity; 
ec,  ectoderm;  en,  endoderm;  m,  medul- 
lary plate;  mp,  mesodermic  pouch. — 
{Halschtk.) 


rOKMATION  OF   THE   OEBM   lAYEM 


49 


forms  an  invagination  (Fig.  2$,  gc),  the  roof  of  which  is  composed 
of  relatively  small  yollc-containing  cells  wfiile  its  floor  is  formed  by 
the  large  cells  of  the  lower  pole  of  the  blastula.  The  cavity  of  the 
blastula  is  not  sufficiently  large  to  allow  of  the  typical  invagination 
of  all  these  large  cells,  so  that  they  become  enclosed  by  the  rapid 
growth  of  the  ectoderm  cells  of  the  upper  pole  of  the  ovum  over 


Fio.  J5. — Sectiou  thkodgh  a  Gastrcia  of  Amblyslomi. 

dl,  Dorsal  Up  of  blastopore;  gc,  digestive  cavity;  gr,  area  of  mesoderm  formatioii;  i»«, 

mi:aodam.—(EycUshymtr.) 

them.  Before  this  growth  takes  place  the  blastopore  corresponds 
to  the  entire  area  occupied  by  the  large  yolk  cells,  but  later,  as  the 
growth  of  the  smaller  cells  gradually  encloses  the  larger  ones,  it 
becomes  smaller  and  is  finally  represented  by  a  small  opening 
situated  at  what  will  be  the  hind  end  of  the  embryo. 

Soon  after  the  archenteron  has  been  formed  a  solid  plate  of  cells, 
eventually  splitting  into  two  layers,  arises  from  its  roof  on  each  side 
of  the  median  line  and  grows  out  into  the  space  between  the  ecto- 
derm and  endoderm  (Fig.  26,  «*'  and  w*'),  evidently  corresponding 
to  the  hollow  plates  formed  in  the  same  situations  in  Amphioxus. 

4 


so 


VOUIATION  or  TRB  OIUI  IAYIB8 


This  is  not,  however,  the  only  source  of  the  mesoderm  in  the  am- 
phibia, for  while  the  blastopore  is  still  quite  large  there  may  be 
found  surrounding  it,  between  the  endoderm  and  ectoderm,  a  ring  of 
mesodermal  tusue  (Fig.  as,  mes).  As  the  blastopore  diminishes  in 
siie  and  ita  lips  come  together  and  unite,  the  ring  of  mesoderm 
forms  first  an  oval  and  then  a  band  lying  beneath  the  line  of  closure 
of  the  blastopore  and  united  with  both  the  superjacent  ectoderm 
and  the  subjacent  endoderm.    This  line  of  fusion  of  the  three  germ 


Fio.  26.— SiciKiii  TBwniGR  AM  EtOKYO  Akpbhian  (T«itok)  or  i4  Daw.  mownro 
IBS  FoncATioH  or  tbz  Gastml  Misoouui. 
•»,  Ectoderm;  ch,  chords  endoderm;  lU,  digestive  cavity;  M,  endoderm;  mi'  and 
•M",  loinatic  and  spianclmic  layer*  of  the  mesoderm.    D,  donal  and  V  TentraL— 
iatftwig,) 

layers  is  known  as  the  primi'-e  streak.  It  is  convenient  to  distin- 
guish the  mesoderm  of  the  primitive  streak  from  that  formed  from 
the  dorsal  wall  of  the  archenteron  by  speaking  of  the  former  as  the 
prostomiai  and  the  latter  as  the  gastral  mesoderm,  though  it  must  be 
understood  that  the  two  are  continuous  immediately  in  front  of  the 
definitive  blastopore. 

In  the  reptilia  still  greater  modifications  are  found  in  the  method 
of  formation  of  the  germ  layers.  Before  the  enveloping  cells  have 
completely  surrounded  the  yolk-mass,  a  cresrentic  groove,  resembling 
that  occurring  in  amphibia,  appears  near  the  posterior  edge  of  the 


rosiunoN  oi  the  ozsm  iaviu 


S' 


biMtoderm,  the  cells  of  which,  in  front  of  the  groove,  arrange  them- 
selves in  a  superficial  layer  one  cell  thick,  which  may  be  regarded  as 
the  ectoderm  (Fig.  ay,  ec),  and  a  subjacent  mass  of  somewhat 
scattered  cells.  Later  the  lowermost  cells  of  this  subjacent  mass 
arrange  themselves  in  a  continuous  layer,  constituting  what  is  termed 
the  primary  mdoderm  («,'),  while  the  remaining  cells,  aggregated 


|^^j^v^ggq> 


1 10.  27.-Lo»omn)raAi Skhoms TmonoH Blastodimh  of ihiGecko, showdto 
Gastrulation. 

«,  Ectoderm;  «.,  wondary  mdoderm;  «•',  primary  endoderm;  fm,  prostomial  meso- 
derm.— {WiU.) 

especially  in  the  region  of  the  crescentic  groove,  form  the  prostomial 
mesoderm  {^m).  In  the  region  -nclosed  by  the  groove  a  distinct 
delimitation  of  the  various  layers  does  not  occur,  and  this  region 
forms  the  primitive  streak.  The  groove  now  begins  to  deepen, 
formmg  an  invagination  of  secondary  endoderm,  the  extent  of  this 
•invagination  being,  however,  very  different  in  different  species 
In  the  gecko  (WUl)  it  pushes  forward  between  the  ectoderm  and 
primary  endoderm  almost  to  the  anterior  edge  of  the  blastoderm 
(I'lg.  27,  B),  but  later  the  cells  forming  its  floor,  together  with  those 


s» 


rOSKATION  or   THE  OUM   LAVCM 


of  the  primary  endoderm  Immediately  below,  undergo  a  degenera- 
tion, the  roof  cells  at  the  tip  and  lateral  margins  of  the  invagination 
becoming  continuous  with  the  persisting  portions  of  the  primary 
endoderm  (Figs,  ay,  C  and  a8,  B).  This  layer,  following  the  envelop- 
ing cells  in  their  growth  over  the  yolk-mass,  gradually  surrounds 
that  structure  so  that  it  comes  to  lie  within  the  archenteron.  In 
some  turtles,  on  the  other  hand,  the  disappearance  of  the  floor  of  the 
invagination  takes  place  at  a  very  early  stage  of  the  infolding,  the 


FlO.  38. — DlAGIAHS  ILLCSTIATINO  THE  FOUATION  Of  TBI  GABnAL  MiSODXIK  IN 

THx  Gecko. 

M,  Chorda  endoderm;  te,  ectoderm;  m,  secondary  endoderm;  «■',  primary  endoderm; 

gm,  gastral  mesoderm. — {WiU.) 

roof  cells  only  persisting  to  grow  forward  to  form  the  dorsal  wall  of 
the  archenteron.  This  interesting  abbreviation  of  the  process 
occurring  in  the  gecko  indicates  the  mode  of  development  which  is 
found  in  the  mammalia. 

The  existence  of  a  prostomial  mesoderm  in  connection  with  the 
primitive  streak  has  already  been  noted,  and  when  the  invagination 
takes  place  it  is  carried  forward  as  a  narrow  band  of  cells  on  each 
side  of  the  sac  of  secondary  endoderm.  After  the  absorption  of  the 
ventral  wall  of  the  invagination  a  folding  or  turning  in  of  the  margins 


rOKMATION  or  THE   OESM  LAVEkS 


53 


of  the  Mcondary  endoderm  occun  (Fig.  38),  whereby  it»  lumen 
become*  reduced  in  size  and  it  passes  off  on  each  side  into  a  double 
plate  of  cells  which  constitute  the  gastral  mesoderm.    Later  these 


.  Fio  30 SECnoNS  or  Ova  op  a  Bai  showing  (.4)  thi  Foimation  oi  ths  Ehso- 

DEM  AND  (B  AND  C)  OT  THE  AMNIOTIC  CaVITV.— ( VoM  BenaUn.) 

plates  separate  from  the  archcnteron  as  in  the  lower  forms.    All  the 
prostomial  mesoderm  does  not,  however,  arise  from  the  primitive 


54 


rOUUTION  OF  THE  G£K1C  lAVEIS 


Streak  region,  but  a  considerable  amount  also  h&j  its  origin  from 
the  ectoderm  covering  the  yolk  outside  the  limits  of  the  blastoderm 
proper,  a  mode  of  origin  which  serves  to  explain  the  phenomena  later 
to  be  described  for  the  mammalia. 

In  comparison  with  the  amphibians  and  Amphioxus,  the  reptilia 
present  a  subordination  of  the  process  of  invagination  in  the  forma- 
tion of  the  endoderm,  a  primary  endoderm  making  its  appearance 
independently  of  an  invagination,  and,  in  association  with  this 
subordination,  there  is  an  early  appearance  of  the  primitive  streak, 
which,  from  analogy  with  what  occurs  in  the  amphibia,  may  be 
assumed  to  represent  a  portidn  of  the  blastopore  which  is  closed 
from  the  very  beginning. 

Turning  now  to  the  mammalia,  it  will  be  found  that  these 
peculiarities  become  still  more  emphasized.  The  inner  cell-mass 
of  these  forms  corresponds  to  the  blastoderm  of  the  reptilian  ovum, 
and  the  first  differentiation  which  appears  in  it  concerns  the  cells 
situated  next  the  cavity  of  the  vesicle,  these  cells  differentiating  to 
form  a  distinct  layer  which  gradually  extends  so  as  to  form  a  com- 
plete lining  to  the  inner  surface  of  the  enveloping  cells  (Fig.  29,  A). 
The  layer  so  formed  is  endodermal  and  corresponds  to  the  primary 
endoderm  of  the  reptiles. 

Before  the  extension  of  the  endoderm  is  completed,  however, 
cavities  begin  to  appear  in  the  cells  constituting  the  remainder  of  the 
inner  mass,  especially  in  those  immediately  beneath  Rauber's  cells 
(Fig.  29,  B),  and  these  cavities  in  time  coalesce  to  form  a  single 
large  cavity  bounded  above  by  cells  of  the  enveloping  layer  and 
below  by  a  thick  plate  of  cells,  the  embryonic  disk  (Fig.  29,  C).  The 
cavity  so  formed  is  the  amniotic  cavity,  whose  further  history  will  be 
considered  in  a  subsequent  chapter. 

It  may  be  stated  that  this  cavity  varies  greatly  in  its  development  in 
different  mammals,  being  entirely  absent  in  the  rabbit  at  this  stage  of 
development  and  reaching  an  excessive  development  in  such  forms  as 
the  rat,  mouse,  and  guinea-pig.  The  condition  here  described  is  that 
which  occurs  in  the  bat  and  the  mole,  and  it  seems  probable,  from  what 
occurs  in  the  youngest  human  embryos  hitherto  observed,  that  the  proc- 
esses in  man  are  closely  similar. 


fobmahon  of  the  geui  layebs 


55 


While  these  changes  have  been  taking  place  a  splitting  of  the 
enveloping  layer  has  occurred,  so  that  the  wall  of  the  ovum  is  now 
formed  of  three  layers,  an  outer  one  which  may  be  termed  the 
irotihoUast,  a  middle  one  wliich  probably  is  transformed  into  the 
extra-embryonic  mesoderm  of  later  stages,  though  its  significance 
is  at  present  somewhat  obscure,  and  an  inner  one  which  is  the 


Ml 


Fio.  io.—A,  Side  Vnsw  of  Ovdm  of  Rabbit  Seven  Days  Ou)  (KMittr)-  B 
EiiBEYONic  Disi  OF  A  MoLE  (Htaft);  C,  EiiBivoNic  Disk  of  a  Doo's  Ovuic  of 
ABOUT  Fifteen  Days  (Bwmrt).  vv,»vy,u«or 

ed,  Embiyonic  disk;  hn,  Hemcn's  node;  mg,  medullary  groove;  ps,  primiUve  atieak- 
vo,  vascular  area.  ' 

primary  endoderm.  In  the  bat,  of  whose  ovum  Fig.  29,  C,  repre- 
sents a  section,  that  portion  of  the  middle  layer  which  forms  the 
roof  of  the  amniotic  cavity  disappears,  only  the  trophoblast  per- 
sisting in  this  region,  but  in  another  form  this  is  not  the  case,  the 
roof  of  the  cavity  being  composed  of  both  the  trophoblast  and  the 
middle  layer. 

A  rabbit's  ovum  in  which  there  is  yet  no  amniotic  cavity  and  no 
splitting  of  the  enveloping  layer  shows,  when  viewed  from  above, 


S6 


FOKlfATION   OF   THE    GERM   LAYESS 


a  relatively  small  dark  area  on  the  surface,  which  is  the  embryonic 
disk.  But  if  it  be  looked  at  ifrom  the  side  (Fig.  30,  A),  it  will  be  seen 
that  the  upper  half  of  the  ovum,  that  half  in  which  the  embryonic 
disk  occurs,  is  somewhat  darker  than  the  lower  half,  the  line  of 
separation  of  the  two  shades  corresponding  with  the  edge  of  the 
primary  endoderm  which  has  extended  so  far  in  its  growth  around 
the  inner  surface  of  the  enveloping  layer.  A  little  later  a  dark  area 
appears  at  one  end  of  the  embryonic  disk,  produced  by  a  prolifera- 
tion of  cells  in  this  region  and  having  a  somewhat  crescentic  form. 
As  the  embryonic  disk  increases  in  size  a  longitudinal  band  makes 
its  appearance,  extendlni;  forward  in  the  median  line  nearly  to  the 
center  of  the  disk,  and  represents  the  primitive  streak  (Fig.  30,  B), 
a  slight  groove  along  its  median  line  forming  what  is  termed  the 
primitive  groove.  In  slightly  later  stages  an  especially  dark  spot 
may  be  seen  at  the  front  end  of  the  primitive  streak  and  is  termed 
Hensen's  node  (Fig.  30,  C,  An),  while  still  later  a  dark  streak  may 
be  observed  extending  forward  from  this  in  the  median  line  and  is 
termed  the  head-process  of  the  primitive  streak. 


Fio.  ji. — ^PosTEBun  Portion  or  A  1.0NGrTUDiNAL  Section  tbkough  tbz  Ehbbyonic 

Disk  or  A  Mole. 
bl,  Blastopore,  ec,  ectoderm;  m,  endodeim;  firm,  prostomial  mesoderm. — (After  Heafe.) 


1 


To  understand  the  meaning  of  these  various  dark  areas  recourse 
must  be  had  to  the  study  of  sections.  A  longitudinal  section  through 
the  embryonic  disk  of  a  mole  ovum  at  the  time  when  the  crescentic 
area  makes  its  appearance  is  shown  in  Fig.  31.  Here  there  is  to  be 
seen  near  the  hinder  edge  of  the  disk  what  is  potentially  an  opening 
(bl),  in  front  of  which  the  ectoderm  (ec)  and  primary  endoderm  (en) 
can  be  clearly  distinguished,  while  behind  it  no  such  distinction  of 


FORUATION   OF   THi;    GERM   LAYEBS 


57 


the  two  layers  is  visible.  This  stage  may  be  regarded  as  compar- 
able to  a  stage  immediately  preceding  the  invagination  stage  of 
the  reptilian  ovum,  and  the  region  behind  the  blastopore  will 
correspond  to  the  reptilian  primitive  streak.  The  later  forward 
extension  of  the  primitive  streak  is  due  to  the  mode  of  growth  of  the 
embryonic  disk.  Between  the  stages  represented  in  Figs.  31  and 
30,  B,  the  disk  has  enlarged  considerably  and  the  primitive  streak 
has  shared  in  its  elongation.  Since  the  blastopore  of  the  earlier 
stage  is  situated  immediately  in  front  of  the  anterior  extremity  of 
the  primitive  streak,  the  point  corresponding  to  it  in  the  older  disk 
is  occupied  by  Hensen's  node,  this  structure,  therefore,  representing 
a  proliferation  of  cells  from  the  region  formerly  occupied  by  the 
blastopore. 


Fio.  3J.— Tkansvikse  Section  op  the  Ehbkyonic  Abea  op  a  Doo's  Ovbm  at  aboct 

THE  Stage  op  Development  shown  in  Fio.  39,  C. 

The  section  passies  through  the  head  process  {Chp);  it,  mesoderm.— (Bo»w<.) 

As  regards  the  head  process,  it  is  at  first  a  solid  cord  of  cells 
which  grows  forward  in  the  median  line  from  Hensen's  node,  lying 
between  the  ectoderm  and  the  primary  endoderm.  Later  a  lumen 
appears  in  the  center  of  the  cord,  forming  what  has  been  termed  the 
chorda  canal,  and,  in  some  forms,  including  man,  the  canal  opens  to 
the  surface  at  the  center  of  Hensen's  node.  The  cord  then  fuses 
with  the  subjacent  primary  endoderm  and  then  opens  out  along  the 
line  of  fusion,  becoming  thus  transformed  into  a  flat  plate  of  cells 
continuous  at  either  side  with  the  primary  endoderm  (Fig.  32,  Chp). 
The  portion  of  the  chorda  canal  which  tiaverscs  Hensen's  node  now 


S8 


FOSUATION  or  IHE   GEBK  LAYEB8 


opens  below  into  what  will  be  the  primitive  digestive  tract  and  is 
tenned  the  neurtnteric  canal  (Fig.  33,  nc);  it  eventually  closes  com- 
pletely, being  merely  a  transitory  structure.  The  similarity  of  the 
head  process  to  the  invagination  which  in  the  reptilia  forms  the 
secondary  endoderm  seems  clear,  the  only  essential  difference  being 
that  in  the  mammalia  the  head  process  arises  as  a  solid  cord  which 
subsequently  becomes  hollow,  instead  of  as  an  actual  invagination. 
The  difference  accounts  for  the  occurrence  of  Hensen's  node  and 
also  for  the  mode  of  formation  of  the  neurenteric  canal,  and  cannot 
be  considered  as  of  great  moment  since  the  development  of  what  are 
eventually  tubular  structures  («.  g.'.,  glands)  as  solid  cords  of  cells 
which  subsequently  hollow  out  is  of  common  occurrence  in  the 
mammalia.  It  should  be  stated  that  in  some  mammals  apparently 
the  most  anterior  portion  of  the  roof  of  the  archenteron  is  formed 
directly  from  the  cells  of  the  primary  endoderm,  which  in  this  region 
are  not  replaced  by  the  head  process,  but  aggregate  to  form  a  compact 
plate  of  cells  with  which  the  anterior  extremity  of  the  head  process 


Flo-  33-— DiAOiAM  01  A  LoNorruDiNAi.  SicnOM  thsouoh  TBS  Ehbeyootc  Disk  ot 

A  Mole. 
am.  Amnion;  ct  chorda  endoderm;  ec,  ectoderm;  nc,  neurenteric  canil-  ps  primitive 
stre»k.— (H«^.)  ' 

unites.    Such  a  condition  would  represent  a  further  modification  of 
the  original  condition. 

A«  regards  the  formation  of  the  mesoderm  it  is  posable  to  rec- 
ognize both  the  prostomial  and  gastral  mesoderm  in  the  mammalian 
ovum,  though  the  two  parts  are  not  so  clearly  distingubhable  as  in 
lower  forms.  A  mass  of  prostomial  mesoderm  is  formed  from  the 
primitive  streak,  and  when  the  head  process  grows  forward  it  cames 


roiuAnoN  OF  the  oeui  iavxbs 


59 


with  it  some  of  this  tissue.  But,  in  addition  to  this,  a  contribution  to 
the  mesoderm  is  also  apparently  furnished  by  the  cells  of  the  head 
process,  in  the  form  of  lateral  plates  situated  on  each  side  of  the 
middle  line.    These  plates  are  at  first  solid  (Fig.  34,  gm),  but  their 


Fio.  34.— Tjansvirsi  SlcnoK  rmtoDOH  the  EionYoinc  Disk  of  a  Rabbit. 
ck.  Chord*  CBdoderm; «,  ectoderm;  m,  endoderm;  fm,  gastral  mesoderm.— (/4/i<r  i 
Bentdm.) 


f>0.  aS^DlASXAXS  IlXtlSIHAIINO  THI  RiLATIONS  OF  IHI  ChICI  EhbIYO  TO  TH» 

Pinnnvi  Strmk  at  Dufebint  Stages  of  Devilopiieht.— (P«Wm.) 

cells  quickly  arrange  themsdves  in  two  layers,  between  which  a 
r^lomic  space  later  appe-rs. 

Furthermore,  as  has  already  been  pointed  out,  the  layer  of 


'IMI 


6o 


SIGNIFICANCE   OF   THE    GERM   LAVESS 


enveloping  cells  splits  into  two  concentric  layers,  the  inner  of  which 
seems  to  be  mesodermal  in,  its  nature  and  forms  a  layer  lining  the 
interior  of  the  trophoblast  and  lying  between  this  and  the 
primary  endoderm.  TJiis  layer  is  by  no  means  so  evident  in  the 
lower  forms,  but  is  perhaps  represented  in  the  reptilian  ovum  by  the 
cells  which  underlie  the  ectoderm  in  the  regions  peripheral  to  the 
blastoderm  proper  (see  p.  54). 

It  has  been  experimentally  determined  (Assheton,  Peebles)  that  in 
the  chick,  whose  embryonic  disk  presents  many  features  similar  to  those 
of  the  mammalian  ovum,  the  central  point  of  the  unincubated  disk  corre- 
sponds to  the  anterior  end  of  the  primitive  streak  and  to  the  point  situated 
immediately  behind  the  heart  of  the  later  embiyo  and  immediately  in 
front  of  the  first  mesodermic  somite  (see  p.  77),  as  shown  in  Fig.  35.  If 
these  results  be  regarded  as  applicable  to  the  human  embryo,  then  it 
may  be  supposed  that  in  this  the  head  region  is  developed  from  the 
portion  of  the  embryonic  disk  situated  in  front  of  Hensen's  node,  while 
the  entire  trunk  is  a  product  of  the  region  occupied  by  the  node. 

The  Significance  of  the  Germ  Layers. — The  formation  of 
the  three  germ  layers  is  a  process  of  fundamental  importance,  since 
it  is  a  differentiation  of  the  cell  units  of  the  ovura  into  tissues  which 
have  definite  tasks  to  fulfil.  As  has  been  seen,  the  first  stage  in  the 
development  of  the  layers  is  the  formation  of  the  ectoderm  and 
endoderm,  or,  if  the  physiological  nature  of  the  layers  be  considered, 
it  is  the  differentiation  of  a  layer,  the  endoderm,  which  has  princi- 
pally nutritive  functions.  In  certain  of  the  lower  invertebrates,  the 
class  Coelentera,  the  differentiation  does  not  proceed  beyond  this 
diploblastic  stage,  but  in  all  higher  forms  the  intermediate  layer  is 
also  developed,  and  with  its  appearance  a  further  division  of  the 
functions  of  the  organism  supervenes,  the  ectodeirm,  situated  upon 
the  outside  of  the  body,  assuming  the  relational  functions,  the 
endoderm  becoming  still  more  exclusively  nutritive,  while  the  remain- 
ing functions,  supportive,  excretory,  locomotor,  reproductive,  etc., 
are  assumed  by  the  mesoderm. 

The  manifold  adaptations  of  development  obscure  in  certain 
cases  the  fundamental  relations  of  the  three  layers,  certain  portions 
of  the  mesoderm,  for  instance,  failing  to  differentiate  simultaneously 


SIGNIFICANCE   01   THE   OEBM   LAYERS 


6l 


with  the  rest  of  the  layer  and  appearing  therefore  to  be  a  portion  of 
either  the  ectoderm  or  endoderm.  But,  as  a  rule,  the  layers  are 
structural  units  of  a  higher  order  tlmn  the  cells,  and  since  each 
assumes  definite  .physiological  functions,  definite  structures  have 
their  origin  from  each. 
Thus  from  the  ectoderm  there  develop: 

1.  The  epidermis  and  its  appendages,  hairs,  nails,  epidermal 
glands,  and  the  enamel  of  the  teeth. 

2.  The  epithelium  lining  the  mouth  and  the  nasal  cavities,  as 
well  as  that  lining  the  lower  part  of  the  rectum. 

3.  The  nervous  system  and  the  nervous  elements  of  the  sense- 
organs,  together  with  the  lens  of  the  eye. 

From  the  endoderm  develop: 

1.  The  epithelium  lining  the  digestive  tract  in  general,  together 
with  that  of  the  various  glands  associated  with  it,  such  as  the  liver 
and  pancreas. 

2.  The  lining  epithelium  of  the  larynx,  trachc?,  and  lungs. 

3.  The  epithelium  of  the  bladder  and  urethra  (in  part). 
From  the  mesoderm  there  are  formed: 

I.  The  various  connective  tissues,  including  bone  and  the  teeth 
(except  the  enamel). 

3.  The  muscles,  both  striated  and  non-striated. 

3.  The  circulatory  system,  including  the  blood  itself  and  the 
lymphatic  system. 

4.  The  lining  membrane  of  the  serous  cavities  of  the  body. 

5.  The  kidneys  and  ureters. 

6.  The  internal  organs  of  reproduction. 

From  this  list  it  will  be  seen  that  the  products  of  the  mesoderm 
are  tnore  varied  than  those  of  either  of  the  other  layers.  Among 
its  products  are  organs  in  which  in  either  the  embryonic  or  adult 
condition  the  cells  are  arranged  in  a  definite  layer,  while  in  other 
structures  its  cells  are  scattered  in  a  matrix  of  non-cellular  material, 
as,  for  example,  in  the  connective  tissue,  bone,  cartilage,  and  the 
blood  find  lymph.  It  has  been  proposed  to  distinguish  these  two 
forms  of  mesoderm  as  mesothelium  and  mesenchyme  respectively, 


63 


URBATUU 


a  distinctioii  which  is  undoubtedly  convenient,  though  probably  de- 
void of  the  fundamental  importance  which  has  been  attributed  to  it 
by  some  embtyologists. 


LITERATURE. 

R.  AssaiTON:  "The  ReinTeiligadoa  into  the  Ettfy  Stag«  at  the  Devdopnunt  d 

the  Ribblt,"  QaarUrfy  Joutn.  tfUierm.  Sciaut,  xxxvn,  1894. 
R.  Assbxton:  "The  Development  of  the  Pig  During  the  Pint  Ten  Diyi,"  QiiarUrly 

Joun.  o/Uicrose.Scitna,  xu,  1898. 
R.  AasBnoN:  "The  Segmentation  of  the  Ovum  of  the  Sheep,  with  Obaervitioni  on 

the  Hypotheib  of  *  HypoUutic  Origin  for  the  Trophoblut,"  QiurUrly  Journ. 

of  UicroK.Scimet,  XLS,  189S. 
E.  VAN  Binidih:  "  Recherchei  tur  les  pfcmien  ittdei  du  cUvelc^pement  du  Muiin 

(VeipertUio  murintu),"  AmUom.  Anuittr,  xn,  1899. 
R.  BoHKn:  "BdtiSge  lur  Embi)rol<^  der  WiederUuer  gewonnen  am  Schafei," 

ArcUvjar  Anat.  wid  PkysM.,  Anal.  AUk.,  1884  and  1889. 
R.  Bohnit:  "Bdtitge  nil  Embiyologie  des  Hundes,"  Anat.  BefU,  a,  1897. 
G.  Bokn:  "Ente  Entwidcelungsvorgknge,"  ErgibmsM  dm  Atut.  tmd  BnlwIeUimgs- 

fch.,  1, 1892. 

E.  G.  Cohkuh:  "The  Cauw  of  Invene-S]nnmetr)r,"  Aiulam.  Anuigw,  xxm,  1903. 

A.  C.  ExcLESKTiaa:  "The  Earijr  Derelopment  of  Amblyitoma  with  Obeerraikau 

on  Some  Other  Vertebiatra,"  Jmrn.  o/ltcrtkol.,  x,  1895. 

B.  Haiscbik:  "Stndioi  aber  Entwiddung  des  Amphioxus,"  Arbtiten  out  dtm  tookt 

InM.  m  Wim,  iv,  iSSi. 
W.  Hun:  "The  Devdopment  of  the  Mole  (Talpa  eunpca),"  Qwrlwly  Jam.  of 

liicro«.ScUnet,  xxm,  1883. 
A.  A.  W.  HinucBi:  "Studies  on  Mammalian  Embrjrology  II:  The  Derdopment 

of  the  Gcimlnal  Layen  of  Sorex  ralgaris,"  Quartwlf  Journ.  of  Uiaox.  Scimct, 

xxn,  189a. 

F.  Kxibxl:  "Stttdion    air    EntwicUungsgesdiidite    des    Schwlies,"    Mcrflulog. 

Arbeitm,  va,  1893. 
F.  Kubel:  "Die  Gastrulatiaa  und  die  Keimblattbildung  der  Wfarbdtiere,"  Ergtbmsse 

it  Anat.  und  ExIvicUimtsfsck.,  x,  1901. 
M.  £DNSEii%Ln:  "Die  Eifurdiung  des  Igdt  (Eiinaceus  europnu  L.),"  ZtUschr. 

far  wismuch.  Zool.,  Lxxxv,  1906. 
K.  Mrrsttzmi  and  C.  IsmuwA:  "Ou  the  Formation  of  the  Germinal  Layen  in 

Chelonia,"  Quarlmiy  Jornn.  ofUitrose.Scimce,  xxvn,  rSS?. 
F.  FiKBLzs:  "The  Loottion  of  the  Chick  embrjro  upon  the  Blastoderm,"  Jim.  of 

Exfm.  Zool.,  I,  r904. 

E.  Scunka:  "Sindien  aber  Entwidulungsgesdiichte  der  Thiere,"4tes  Heft,  1886-S7; 
Stes  Heft,  1891-91. 

J.  SosvTiA:  "DieBefrucfatungundFuichungdesEiesdaMaiSi'Miciiir^  nditosk. 
Anat.,  XLV,  1895. 


UTXBATCU 


63 


CHAPTER  III. 

THE  MKDULLARY  GROOVE,  ITOTOCHORD,  AHD  MESO- 
DERMIC  SOMITES. 

In  the  preceding  chapter  the  development  of  the  mammalian 
ovum  has  been  described  up  to  and  including  the  formation  of  the 
three  germinal  layers.  The  earlier  stages  of  development  there 
described  are  practically  unknown  in  the  human  ovum,  but  for  the 
stages  subsequent  to  the  establishment  of  the  germinal  layers 
human  material  is  available,  and  it  will,  therefore,  now  be  con- 
venient to  consider  the  structure  of  the  younger  human  ova  at 
present  known  and  to  trace  in  them  the  appearcnce  and  develop- 
ment of  such  structures  as  the  primitive  streak,  the  head  process  and 
the  gastral  mesoderm. 

The  youngest  human  ovum  at  present  known  is  that  described 
by  Bryce  and  Teacher,  but,  unfortunately,  it  presents  certain 
features  that  are  evidently  abnormal,  so  that  it  becomes  doubtful 
how  far  it  may  be  accepted  as  representing  the  typical  condition. 
The  trophoblast,  which  was  very  thick  and  clearly  differentiated 
into  two  layers,  enclosed  a  space  whose  diameter  was  about  0.63 
mm.  and  which  was  largely  occupied  by  a  loose  syncytial  tissue, 
presumably  mesoderm.  Toward  the  center  of  this  was  an  irregular 
cavity  in  which  were  two  vesicles,  quite  separate  from  one  another 
and  probably  together  representing  the  embryo,  the  smaller  one 
being  the  amniotic  cavity  and  the  larger  one  the  yolk-sac  (Fig.  36). 
The  separation  of  these  two  structures  is  apparently  an  abnormality 
and  it  is  possible  that  the  cavity  in  which  they  lie  is,  as  Bryce  and 
Teacher  suggest,  an  artefact  produced  by  contraction  of  the  syncytial 
mesoderm  during  the  preservation  of  the  ovum. 

If  comparison  of  this  ovum  with  those  of  other  mammals  is 
warranted,  it  may  be  likened  to  that  of  the  bat  as  shown  in  Fig.  29, 

64 


IHE  UXDUUAXY  OKOOVX 


6$ 


C  With  the  difference  that  the  mesoderm  that  lines  the  trophoblast 
n  that  ovum  has  become  much  more  voluminous  and  forms   he 

a^condjuon  that  may  be  represented  diagrammatically  as  in  Fig. 

Somewhat  older  are  the  ova  described  by  Peters  Fetzer   Tnn., 
and  Herzog.    The  Peters  ovum  was  taken '^mrheS.^:;f 


woman  who  had  committed  suicide  one  calendar  month  after  the 

last  menstruation,  and  it  measured  about  x  mm.  in  Zmeter     xt 

entire  mner  surface  of  the  trophoblast  (Fig.  37   c«)  7^1  n^H  h 

aver  of  mesoderm  (en,),  which,  on  the  suffa    'fShlsT  way  from* 

he  utenne  cavity,  was  considerably  thicker  than  eSe'e  foril^ 

n  area  of  attachment  of  the  embryo  to  the  wall  of  the  ovu  °      In 

the  substance  of  this  thickening  was  the  amniotic  caX  (  J^ 

whose  roof  was  formed  by  flattened  cells  which  at  the  ,mI  V   ^' 

condnuous  with  a  layer  of  columnar  ce^stX  th  t-.^'^^X' 

cavty  and  constituting  the  embryonic  ectoderm  (J).    Imm^^ 


66 


1SB  KEtWUAKT  OlOOVX 


below  thit  WM  a  layer  of  mefoderm  (m)  which  iplit  at  the  edge  of 
the  embryonic  disk  into  two  layers,  one  of  which  became  continuous 
with  the  mesodcrmic  thickening  and  so  with  the  layer  of  mesoderm 
lining  the  interior  of  the  trophoblast,  while  the  other  enclosed  a  sac 
lined  by  a  layer  of  endodermal  cells  and  forming  the  yolk-sac  {ys). 
The  total  length  of  the  embryo  was  0.19  mm.,  and  so  far  as  its 
ectoderm  and  mesoderm  are  concerned  it  might  be  described  as  a 


Fio.  37.— SicnoH  or  Emivo  Ami  Aojachit  Foinoif  01  ah  Ovmi  or  i  km. 

tm.  Amniotic  cavity;  m,  chorionic  ectoderm;  cm,  chorionic  moodenn;  m,  embcjronic 

ectoderm;  m,  endoderm;  m,  embrjronic  meaodenn;  ys,  jrolk-Hck.— (PMri.) 


flat  disk  resting  on  the  surface  of  the  yolk-sac,  though  it  must  be 
tmderstood  that  the  yolk-sac  also  to  a  certain  extent  forms  part  of 
the  embryo. 

This  embryo  seems  to  be  in  an  early  stage  of  the  primitive  streak 
tormation,  before  the  development  of  the  head  process.  On  com- 
paring it  with  the  stage  of  development  represented  in  Fig.  38,  A, 
it  will  be  seen  to  present  some  important  advances.  The  cavity 
(Fig.  38,  B,  C)  into  which  the  yolk-sac  projects  is  unrepresented  in 


1HB  MXDnUAiy  OlOOVZ 


67 


Fig.  38,  A.    How  thii  cavity  i.  formed  cm  only  be  conjectured,  but 

1^  ^^^-  "^^ ''  "^  ''^  "*•  »P""^"«  <"  'he  layer  of  c*II 
which  line,  the  interior  of  the  trophoblast  in  the  earlier  stage  (or 
perhaps  by  the  vacuolixaUon  of  the  central  cell,  of  this  layer)  and 
the  subsequent  accumulation  of  fluid  between  the  two  meso- 

the  sue  M  the  human  ovum  u  due  mainly  to  ih.  ra,,;d  ,  .ov  i,  of 
^cavty,  which  a.  futun,  stages  show.  Is  .h.  ..t^vJ^L  v  k 
portion  of  the  body^vity,  the  spUttint  '     ^a.u  ,li.   li.,;    „f    ,9 


Flo.  38.-DttO««  TO  .BOW  m,  P«»*,u  R,lAIH>HSm«  Of  IB.  P«„  n,  ™, 

mesoderm  by  which  it  is  probably  formed  being  the  precocious 
appearance  of  the  typical  splitting  of  the  mesoderm  to  form  the 
embryomc  body-cavity  which,  as  will  be  seen  in  a  subsequent  chap- 

T  Z^'t  ^l^"  ""'^  **  *  '*'"  "*«"  of 'development.  From  now  on 
Ae  trophoblast  and  the  layer  of  mesoderm  lining  it  may  together 
te  spoken  of  as  the  chorion,  the  mesoderm  layer  being  termed  the 
cHorumtc  mesoderm. 

A  little  older  again  than  the  Peters  and  Herzog  ova  are  those 
described  by  Strahl  and  Beneke  and  by  von  Spee  (Embryo  v  H) 
the  chorionic  cavity  of  the  former  two  having  an  average  diameter 


68 


THE  HEDnUA&Y  OSOOVX 


of  about  3.4  mm.,  while  the  corresponding  size  of  the  latter  two  was 
somewhat  less  than  4.0  mm.  Notwithstanding  the  considerable 
increase  in  the  size  of  these  older  ova,  due  to  the  continued  increase 
in  the  size  of  the  extra-embryonic  coelom,  the  embryos  are  but 


'^, 


Fig.  39. — Th^  Ehbkyp  v.  H.  of  von  ^pn.    The  Left  Halt  or  xhb.Chomon  has 

BEEN  Removed  to  show  the  Eubryo. 
0,  Amniotic  cavity;  6,  beUy-fttalk;  ch,  chorion;  d,  yolk-sac;  e,  extra-<mbryonic  aElom; 
A,  embryonic  disk;  s,  diorionic  villus. — {^onSpte) 

little  advanced  beyond  the  stage  shown  by  the  Peters  embryo. 
The  thickening  of  the  chorionic  mesoderm  that  encloseLi  the  amni- 
otic cavity  has  increased  in  size  and  now  forms  a  pedicle,  known  as 
the  belly-stalk  (Fig.  39,  b),  at  the  extremity  of  which  is  the  yolk-sac 


Fio.  40.— Eheeyo  ieou  the  Bsmeke  Ovum,  the  Root  or  the  Ainnonc  Cavity 

BAVINO  BEEN  iUMOVXD. 

From  a  model.    »,  Belly-staUc ;  p-g.,  primitive  groove;  y,  yolk-sac  — (5<r«W  md  Bmeke.) 

(d).  Furthermore,  the  amniotic  cavity  (a)  now  lies  somewhat  excen- 
trically  in  this  pedicle,  being  near  what  may  be  termed  its  anterior 
surface,  and  the  entire  embryo  projects  like  a  papilla  from  the  inner 
surface  of  the  chorion  into  the  extra-embryonic  coelom.    Fig.  40  is 


1EE  ICEDULLAKY   GSOOVE 


69 


from  a  model  of  the  Beneke  embryo,  detached  from  the  chorion  by 
cutting  through  the  belly-stalk,  and  with  the  roof  of  the  amniotic 
cavity  removed.  The  dorsal  surface  of  the  embryo,  thus  exposed, 
is  an  oval  disk,  resting,  as  it  were,  on  the  yolk-sac,  and  quite  smooth 
except  for  a  slight  longitudinal  groove  upon  its  posterior  portion. 
This  is  the  primUive  groove  and  sections  passing  through  it  show  the 
trimitive  sttcak,  consisting  of  a  sheet  of  mesoderm  interposed 
between  the  ectoderm  and  endoderm,  as  in  the  Peters  embryo,  and 
but  poorly  defined  from  the  other  two  layers.  From  its  anterior 
edge  a  median  process  extends  forward  for  a  short  distance  and  is 
the  head  process  (see  p.  56).  In  front  and  to  the  sides  of  this  there 
is  as  yet  no  mesoderm  intervening  between  the  ectoderm  and 
endoderm. 


Fio.  41.— Embryo  raon  thi  Fjassi  Ovuit,  the  Roof  or  ihe  Ajojiotic  Cavity 

HAVING  BEEN  ReuOVED, 

From  a  model.    I,  bdly-sulk;  p.g.,  primitive  groove;  mg,  medulUiy  groove,  »,  neuren- 
tenc  canal— (Frujji.) 

The  embryonic  disk  of  the  Beneke  embryo  measured  0.75  mm. 
in  length.  That  of  an  embryo  described  by  Frassi  (Fig.  41)  was 
1.17  mm.  in  length,  and  in  correspondence  with  its  greater  size,  it 
presents  some  advances  in  structure  that  are  of  interest.  As  in 
the  younger  embryo  one  sees  a  distinct  primitive  groove  on  the 
posterior  portion  of  the  embryonic  disk,  but  the  groove  terminates 
anteriorly  at  a  distinct  pore  («),  which  perforates  the  disk  and  opens 
ventrally  into  the  yolk-sac.  This  is  the  neurenteric  canal  (see  p.  58) 
and  in  front  of  it  a  groove  extends  forward  in  the  median  line  almost 
to  the  anterior  edge  of  the  embryonic  disk  and  is  evidently  the  first 


''I 

m 


7° 


THE  IIEDULLARY  OROOVK 


indication  of  the  medullary  groove,  whose  walls  are  destined  to  give 
rise  to  the  central  nervous  system.  Sections  passing  through  the 
region  of  the  medullary  groove  show,  lying  beneath  it,  the  head 
process  (Fig.  42,  hp),  already  fused  with  the  endoderm  (compare 
p.  57),  and  on  each  side  of  the  proc  .3S  is  a  plate  of  mesoderm  (gm), 
representing  the  gastral  mesoderm  of  lower  forms  (see  Figs.  a8 
and  34),  but  not  as  yet  showing  any  indications  of  splitting  into  the 
two  layers  that  bound  the  embryonic  coelom  (see  p.  59). 


am 


Fio.  4J.- 


-SZCTIOH  TBXOnSB  THK  FlASSI  EUXVO  JD8T  IN    FxONT  or  THE  NmUM- 

TEUC  Canal. 


Ml,  Amniotic  cavity;  gm,  gutral  mesoderm;  kp,  hemd  process;  mp,  medullary  pbte;  yr. 
yolk-sac. — {Prassi.) 


This  k  just  beginning  to  appear  in  an  embryo,  also  detcribed  by 
von  Spee  and  known  as  embryo  Gl*.  It  measured  f.54  mm.  k 
length  and  is  closely  similar,  in  general  appearance,  to  an  eMlnyo 
described  by  Etemod  and  meuurii^  1.34  mm.  in  len|^  (Fig.  4i)- 
It  diffen  from  the  Frassi  embryo  most  markedly  in  that  the  posterior 
portion  of  the  embryonic  disk,  that  is  to  say  the  primitive  streak 
region,  is  bent  ventrally  so  as  to  lie  almost  at  a  right  angle  with  the 
anterior  portion.  As  a  result  the  belly-stalk  arises  from  the  ventral 
surface  of  the  embryo  instead  of  from  its  p>osterior  extremity,  near 
which  the  opening  of  the  neurenteric  canal  (Fig.  43,»i<;)  is  now  situ- 
ated, almost  the  whole  length  of  the  surface  seen  in  dorsal  view 
being  occupied  by  the  medullary  groove  (m),  which,  in  the  embryo 
Gle,  is  bounded  laterally  by  distinct  ridges,  the  medullary  folds. 


71 


,    .„  ''«'    43— r.MBKYO  I.34IM-LOMO. 

i^rir^lt"-  "TT^    '"■  ''«"y-sWk,  *,  hem;  m,  nedull»»  ™ov.-  „ 


T 


THE   IfEDCLIAKY   FOLDS 


In  the  KrSmer  embryo  Klb  (Fig.  44),  measuring  1.8  mm.  in 
length,  a  new  feature  has  made  its  appearance.  The  medullary  folds 
have  become  quite  high,  and  lateral  to  them  there  is  on  each  side 
a  series  of  five  or  six  oblong  elevations,  which  represent  what  are 
termed  mesodermk  simiUs  and  are  due  to  divisions  of  the  under- 
Ijring  mesoderm. 


\i 


liH 


Fig.  44. — MODD.  OF  THK  KadlBR  ElBKYO  Klh  SDtN  F«OB  THE  DOIISAL  SOIBACK,  THE 

Roop  OF  THX  Ainnonc  CAvmr  bavimg  beek  Rzuovkd. — (Keibei  amd  EiMt.) 

instead  of  proceeding  with  a  description  of  the  external  form  of 
still  older  embryos  it  wiB  be  convenient  to  consider  the  further 
development  of  certain  iSrnctnres  whose  appearance  haB  already 
been  noted,  namely,  tlie  head  process,  the  medullary  folds  and  the 
raesodermic  somites,  and  first  of  ai  the  medullary  folds  may  be 
considered. 

The  Medullary  FoMi.— The  two  folds  are  continuous  antmoriy, 
but  behind  they  are  at  first  seriarate,  the  anterior  portion  of  theiariai- 
tive  streak  l}ring  between  them.  In  forms,  such  as  the  Rcfitilia. 
which  possess  a  distinct  blastopore,  this  caning  lies  in  the  iateTval 
between  the  two,  and  consequently  is  in  the  floor  of  the  medullarv 
groove,  and  in  the  mammalia,  even  though  no  well-defined  blastopore 
is  formed,  yet  at  the  time  of  the  formation  of  the  medullary  fold  an 
opening  breaks  through  at  the  anterior  end  of  the  primitive  streak 
in  the  region  of  Hensen's  node,  and  places  the  cavity  lying  below 
the  endoderm  in  conmiunication  with  the  space  bounded  by  the 
medullary  folds.     The  canal  so  formed  is  termed  the  neurenteric 


THE  KEDCLIASY  FOLDS 


73 


canal  (Figs.  43  and  45,  nc)  and  is  so  called  because  it  unites  what 
will  later  become  the  central  canal  of  the  nervous  system  with  the 
intestine  (enteron).  The  significance  of  this  canal  has  already  been 
discussed  (p.  58);  it  is  of  very  brief  persistence,  closing  at  an  early 
stage  of  development  so  as  to  leave  no  trace  of  its  existence. 


FlO.  45— DmOMH  UK  A  LOWJITUDINAI  SKITION  TBamcSB  THE  EUBXVO  Oe   MlM- 

owNr,  1.54  urn-  m  Lfflaom.  ' 

■(,  AUmtoa:  «*.  amnion;  B,  Wly-stolk;  ■*.  chorio.- A,  aa*t;M,  neurenteric  canal-  V 
— ' =-  villi:  Y,  yolk-Mc— ii«irS#»..)  '     ' 


■'  ''■! 

i 


As  development  proeeetfc  the  medullar}'  fohdR  increase  in  height 
»nd  at  the  same  tiae  indine  toward  one  another  (Fig.  44),  so  that 
their  edges  finaHy  came  iirto  contw-f  iaid  l»«er  lus*  the  two  ecto- 
dermal layers  forming  the  one  uniting  with  the  corresj«s«<fi^  layers 
W  the  oefcer  (Fif  46).  By  t'lis  process  the  medullary  growvf  be- 
comes converted  into  a  medullary  canal  which  later  becomes  the 


74 


nB   NOTOCROID 


central  canal  of  the  spuuK  cord  and  the  ventikiet  at  the  brain,  the 
ectodermal  walls  of  the  canal  thickening  to  give  rise  to  the  central 
nervous  system.  The  closure  of  the  groove  does  not,  however,  talte 
place  simultaneously  along  its  entire  length,  but  begins  in  what 
corresponds  to  the  neck  region  of  the  adult  and  thence  proceeds  both 


Fio.  46. — Duouua  sHowiMO  thi  Mankie  or  tbx  Cuwdu  ot  ibi  Midollut 
Grc^ve. 

anteriorly  and  posteriorly,  the  extension  of  the  fuaon  taking  place 
rather  slowly,  however,  especially  anteriorly,  so  that  an  anterior 
opening  into  the  otherwise  closed  canal  can  be  distinguished  for  a 
considerable  period  (Fig.  53). 

The  Notochord. — While  these  changes  have  been  taking  place  in 
the  ectoderm  of  the  median  line  of  the  embryonic  disk,  modifications 
of  the  subjacent  endoderm  have  also  occiirred.  This  endodf ■  -n, 
it  will  be  remembered,  was  formed  by  the  head  process  of  the  primi- 
tive streak,  and  was  a  plate  of  cells  continuous  at  the  sides  with  the 
primary  endoderm  and  extending  forward  as  far  as  what  will  eventu- 
ally be  the  anterior  part  of  the  pharynx.  Along  the  line  of  its 
jimction  with  the  primary  endoderm  it  gives  rise  to  the  plates  of 
gastral  mesodem  (Fig.  28),  while  the  remainder  of  it  produces  an 


THE   NOTOCHOBS 


75 


u^wrtant  embryonic  organ  known  as  the  nolochord  or  chorda  dorsalis 
and  on  this  account  is  sometimes  termed  the  chorda  mdoderm. 

After  the  separation  of  the  plates  of  gastral  mesoderm  the  chorda 
endoderm,  which  is  at  first  a  flat  band,  becomes  somewhat  curved 
(Fig.  47.  A),  so  that  it  is  concave  on  its  under  surface,  and,  the  curva- 
ture increasing,  the  edges  of  the  plate  come  into  contact  and  finally 
fuse  together  (Fig.  47,  B),  the  edges  of  the  primary  endoderm  at  the 
same  time  uniUng  beneath  the  chordal  tube  so  formed,  so  that  this 
layer  becomes  a  continuous  sheet,  as  it  was  at  its  first  appearance. 


Fro.  47.-T«*Nsv»sE  Sections  nnonoH  Macz  Ehbotos,  saowmo  the  Foiaaioif 
or  THE  NoToeaoK).  — "»™ 

«,  Ectoderm;  en,  endodenn;  m,  mesodenn;  tic  notocbmd.—(Hiapt.) 

The  lumen  which  is  at  first  present  in  the  chordal  tube  is  soon 
obliterated  by  the  enlargement  of  the  cells  which  bound  it,  and 
these  cells  later  undergo  a  peculiar  transformation  whereby  the 
chordal  tube  is  converted  into  a  solid  elastic  rod  surrounded  by  a 
cuticular  sheath  secreted  by  the  cells.  The  notochord  lies  at  first 
immediately  beneath  the  median  line  of  the  medullary  groove,  be- 
tween the  ectoderm  and  the  endoderm,  and  has  on  either  side  of  it 
the  mesodermal  plates.  It  is  a  temporary  structure  of  which  only 
rudiments  persist  in  the  adult  condition  in  man,  but  it  is  a  structure 
characteristic  of  all  vertebrate  embryos  and  per»i»ts  to  a  more  or 
less  perfect  extent  in  many  of  the  fishes,  being  indeed  the  only  axia! 


i  1 


rf 


76 


THE  HESODXUaC  SOUTIS 


n 


skeleton  possessed  by  Amf^ioxus.  In  the  higher  vertebrites  it  is 
almost  completely  replaced  by  the  vertebral  column,  which  develops 
around  it  in  a  nu  nner  to  be  described  later. 

Th*  Metodermic   SomitM.— Turning   now   to    the   middle 
germinal  layer,  it  will  be  found  that  in  it  also  important  changes  take 
place  during  the  early  stages  of  development.    The  probable  mode 
of  development  of  the  extra  embryonic  mesoderm  and  body-cavity 
has  already  been  described  (p.  61)  and  attention  may  now  be  directed 
toward  what  occurs  in  the  «,     ronic  mesoderm.    In  both  the 
Peters  embryo  and  the  emhr:    v.H  described  by  von  Spec  this 
portion  of  the  mesoderm  is    epresented  by  a  plate  of  cells  lying 
between  the  ectoderm  and  endoderm  and  becoming  continuous  at 
the  edges  of  the  embryonic  area  with  both  the  layer  which  surrounds 
the  yolk-sac  and,  through  the  mesoderm  of  the  belly-stalk,  with  the 
chorionic  mesoderm  (Fig.  37).     It  seems  probable,  since  there  is  in 
these  embryos  no  indication  as  yet  of  the  formation  of  the  chorda 
endoderm,  that  this  plate  of  mesoderm  corresponds  to  the  prostomial 
mesoderm  of  lower  forms.    In  older  embryos,  such  as  the  embryo 
Gfe  of  Graf  Spec  and  the  younger  embryo  described  by  Etemod 
(Fig.  43).  the  mesoderm  no  longer  forms  a  continuous  sheet  extend- 
ing completely  across  the  embryonic  disk,  but  is  divided  into  two 
lateral  plates,  in  the  interval  between  which  the  ectoderm  of  the 
floor  of  the  mednlUiy  groove  and  the  chorda  endfiderm  are  in  close 
contact  (Fig.  4^.    These  lateral  plates  represent  the  gastral  meso- 
derm, whose  origin  has  already  been  described  (p.  59),  and  which 
apparently  supplants  the  original  prostomial  mesoderm,  whose 
Jate  in  the  hwnan  embryo  is  at  present  unknown.    The  changes 
which  DOW  occur  have  not  as  yet  been  observed  in  the  human  embryo, 
though  they  probably  resemble  those  described  in  other  mammalian 
embryos,  and  the  phenomena  which  occur  in  the  sheep  may  serve 
to  illustrate  their  probable  nature. 

It  has  been  seen  that  in  the  stage  represented  by  the  Frassi 
enbryo  a  plate  of  mesoderm  has  formed  on  cither  side  of  the  chorda 
endoderm,  aud  that  in  a  Uttr  stage,  represented  by  the  KrtSmer 
embryo  Klb,  a  differentiation  occurs  in  these  plates  leading  to  the 


IRK  lasooniac  somites 


77 


foriMtion  of  mesodermic  somites.  These  make  their  appearance 
m  what  will  later  be  the  cervical  region  of  the  embiyo  and  their 
fotmauon  proceeds  backward  as  the  body  of  the  embryo  increases 
in  length.  A  longitudinal  groove  appears  on  the  dorsal  surface  of 
each  lateral  plate  of  mesoderm,  marking  off  the  more  median  thicker 
portion  from  the  lateral  parts  (Fig.  48),  which  from  this  stage 
onward  may  be  termed  the  ventral  mesoderm.  The  median  or  dorsal 
portions  then  become  divided  transversely  into  a  number  of  more 
or  less  cubical  masses  which  are  termed  the  proloverlebra  or,  better 


r«.  48.-TKAM8V1M.  SimON  ImOUQB  IBt  SlCOm,   MMODIMIC  SOUTI  OF  A 
SRIU  EhSKYO  3  WL  LOMO. 

o^^T^tiT'  "'^'x^'™; '.  ixtermediMe  cdl-imw;  mg,  meduUair  groove-  iw 

mesodermic  somites  (Fig.  48,  ms).  The  cells  of  the  somites  and  of 
the  ventral  mesoderm,  are  at  first  stellate  in  form,  but  later  become 
more  spmdie-shaped,  and  those  near  the  center  of  each  somite  and 
those  of  the  ventral  mesoderm  arrange  themselves  in  regular  layers 
so  as  to  enclose  cavities  which  appear  in  these  regions  (Fig.  48) 
Each  original  lateral  plate  of  gastral  mesoderm  thus  becomes 
divided  longitudinally  into  three  areas,  a  more  median  area  com- 
posed of  mesodermic  somites,  lateral  to  this  a  narrow  area  under- 
lying the  original  longitudinal  groove  which  separated  the  somite 
area  from  the  ventral  mesoderm  and  which  from  its  position  is 
termed  the  intermediate  ceU-mass  (Fig.  48,  /),  and,  finally,  the  ventral 
mesoderm.    This  last  portion  is  now  divided  into  two  layers,  the 


78 


TBx  msoDiaiiic  aoiam 


dorsal  of  which  ii  termed  the  tomalie  mesoderm,  while  the  ventral  one 
is  known  as  the  sphmdmic  mesoderm  (Fig.  48,  jo  and  sp;  and  Fig.  49) , 
the  cavity  which  separates  these  two  layers  being  the  embryonic 
body-cavity  or  fleitroperiUmeai  cavity  (calom),  which  will  eventually 
give  rise  to  the  pleural,  pericardial  and  peritoneal  cavities  of  the  adult 
as  well  as  the  cavity  of  each  tunica  vaginalis  testis. 


rM-49- — TuMivmE  SicnoM  or  ah  Emuyo  or  3.5  hh.  (Sn  Fio.  53)  iBawiiia 
ON  umxft  sn»  ov  ins  MzDTn.LAXY  Canal  a  Mcsodskuc  Sown,  tbx  Ihtix- 
lODun  Ckll-hao,  and  thi  Vintkal  Muodeui.— (Konladhun*.) 

Bqpnning  in  the  neck  re^on,  the  formation  of  the  mesodermic 
somites  proceeds  posteriorly  until  finally  there  are  present  in  the 
human  embryo  thirty-eight  pairs  in  the  neck  and  trunk  regions  of 
the  body,  and,  in  addition,  a  cfrtiin  number  are  developed  in  what 
is  later  the  occipital  region  of  thj  head.  Exactly  how  many  of  these 
occipital  somites  are  developed  is  not  known,  but  in  the  cow  four 
have  been  observed,  and  there  are  reasons  for  believing  that  the 
same  number  occurs  in  the  humau  embryo. 

In  the  lower  vertebrates  a  number  of  cavities  arranged  in  pairs  occur 
in  the  more  anterior  portions  of  the  head  and  have  been  homologized  with 
mesodermic  somities.    Whether  this  homology  be  perfectly  correct  or  not, 


1HX  MZSODEUaC  SOIOTCS  79 

kkZ-^/^.TT*'!."  '"to  somite.,  and  although  pracUcally  nothing 
tate  S^  ?^d*f  "r* '" '?';  •"'™'"  "■"b-yofyet';  from  the  relation! 
farl«^i-^  ^  '"  *•  "*^»'  "«"'"  ""*  muscuUture  in  the  lower 
lorma,  tUere  u  reawn  to  luppoM  that  they  are  not  entirely  unrepresented 


FlO.  50.-T.AOTVM8.  SlCTIOK  OF  AN  ElfB.VO  OF  4-25  KM.  AT  IB.  LEVEL  OF  THE  A,UI 

JtvromEMT. 

The  mesodermic  somites  in  the  earliest  human  embryos  in 
which  they  have  been  observed  contain  a  completely  closed  cavity 
and  this  is  true  of  the  majority  of  the  somites  in  such  a  form  as  the 
sheep.  In  the  four  first-formed  somites  in  this  species,  however 
tht  sonute  cavity  is  at  first  continuous  with  the  pleuroperitoneal 


•waocofr  nsoturiON  rat  chait 

(ANSI  ond  ISO  TEST  CHART  No.  2) 


IM 


lit 


12.0 


m^tA 


/^PLIED  IIVUGE     In 


teSJ  Cott  Mam  SIrMi 


>   rorti         14609        USA 


(716)  «2  -  0300  -  Phon* 
(716)  28B-59B9  -Fa. 


8o 


THE  UESOOEUac   SOMITES 


cavity  and  only  later  becomes  separated  from  it,  and  in  lower  verte- 
brates this  continuity  of  the  somite  cavities  with  the  general  body- 
cavity  is  the  rule.  The  somite  cavities  are  consequently  to  be 
regarded  as  portions  of  the  general  pleuropcritoneal  cavity  which 
have  secondarily  been  separated  off.  They  are,  however,  of  but 
short  duration  and  early  become  filled  up  by  spindle-shaped  cells 
derived  from  the  walls  of  the  somites,  which  themselves  undergo  a 
differentiation  into  distinct  portions.  The  cells  of  that  portion  of  the 
wall  of  each  somite  which  is  opposite  the  notochord  become  spindle- 
shaped  and  grow  inward  toward  the  median  line  to  surround  the 
notochord  and  central  nervous  system,  and  give  rise  eventually  to 
the  lateral  half  of  the  body  of  a  vertebra  and  the  corresponding 
portion  of  a  vertebral  arch.  This  portion  of  the  somite  is  termed  a 
sclerotome  (Fig.  50,  5),  and  the  remainder  forms  a  muscle  plate  or 
myotome  (U)  which  is  destined  to  give  rise  to  a  portion  of  the  volun- 
tary musculature  of  the  body.  The  outer  wall  of  the  somite  has 
been  generally  believed  to  take  part  in  the  formation  of  the  cutis 
layer  of  the  integument  and  hence  has  been  termed  the  ctUis  plate 
or  dermatome,  but  it  seems  probable  that  it  becomes  entirely  trans- 
formed into  muscular  tissue.  '^ 

The  intermediate  cell-mass  in  the  human  embryo,  as  in  lower 
forms,  partakes  of  the  transverse  divisions  which  separate  the  individ- 
ual mesodermic  somites.  From  one  portion  of  the  tissue  in  most  of 
the  somites  (Fig.  50,  Pn)  the  provisional  kidneys  or  Wolffian  bodies 
develop,  this  portion  of  each  mass  being  termed  a  nephrolome,  while 
the  remaining  portion  gives  rise  to  a  mass  of  ceils  showing  no  tend- 
ency to  arrange  themselves  in  definite  layers  and  constituting  that 
form  of  mesoderm  which  has  been  termed  mesenchyme  (see  p.  61). 
These  mesenchymatous  masses  become  converted  into  connective 
tissues  and  blood-vessels. 

The  ventral  mesoderm  in  the  neck  and  trunk  regions  never 
becomes  divided  transversely  into  segments  corresponding  to  the 
mesodermic  somites,  differing  in  this  respect  from  the  other  portions 
of  the  gastral  mesoderm.  In  the  head,  however,  that  portion 
of  the  middle  layer  which  corresponds  to  the  ventral  mesoderm  of 


THE   UESODERiac   SOMITES  g, 

the  development  of  the  branchial  arches  and  clefts  (see  p.  go)  A 
consideration  of  these  segments,  which  are  known  a    the  i.« 

w  h  the  development  of  the  cranial  muscles  and  nerves,  and  in  wha^ 
follows  here  attention  will  be  confined  to  what  occurs  in  the  vemra 
mesoderm  of  the  neck  and  trunk. 

.J1  '^^T}"''  '*y"  (Fig.  51,  vm).  applies  itself  closely  to  the 
endodermal  digestive  tract,  which  is  constricted  off  from  the  dorS 
porfon  o   the  yolk-sac,  and  becomes  converted  into  me  enct" 

The  cells  which  hne  the  pleuroperitoneal  cavity,  however  retain 
heir  arrangement  in  a  layer  and  form  a  part  of  the  serous  HnC" 

iewTor'l  r':''"  """^  ^^^'"^^'  "^^  ---"der  of  the  'ing 
being  formed  by  the  corresponding  cells  of  the  somatic  layer-  and 

Where  the  splanchnic  layer  passes  into  the  somatic,  and  in  close 
proximity  to  the  nephrotome  of  the  intermediate  celi:mass  becom! 
columnar  m  shape  and  are  converted  into  reproductive  ceMs 

The  somauc  layer,  if  traced  peripherally,  becomes  continuous 
at  he  sides  with  the  layer  of  mesoderm  which  lines  the  outer  surface 

i  y  s~Th!t*  '?  ^"'^^P"^'--'^  -">  ">e  mesoderm  JiZ 
belly  stalk.     That  portion  of  it  which  lies  within  the  body  of  the 

layer  o  the  pleuroperitoneum,  becomes  converted  into  mesenchyme 

tw^  zoiTs'aTor  "''^  '"'^  "'  '""^  '^  "'^^^'y  differentiated'nto 
two  zones,  a  more  compact  dorsal  one  which  may  be  termed  the 

wtuch  IS  termed  the  membrana  reuniens  (Fig.  51).    In  the  eariier 

SL  ^hT'^  '^T  ''"'''  '''''  not^xLVventrallJVe;'  d 
LMk  J  .^  ^'''''  "^^"8'^  *^  '™''  •'"ds  and  it  grows  out  into 
these  buds  to  form  an  axial  core  for  them,  in  which  latfr  the  skel  "on 
of  the  limb  forms  The  remainder  of  the  mesoderm  lining  the  sides 
and  ventral  portions  of  the  body-wall  is  at  first  formed  from  the 
membrana  reun.ens,  but  as  development  proceeds  the  somatic 


83 


THE  UESODEKMIC   SOMITES 


layer  gradually  extends  more  ventrally  and  displaces,  or,  more 
properly  speaking,  assimilates  into  itself,  the  membrana  reuniens 
until  finally  the  latter  has  completely  disappeared. 

It  is  to  be  noted  that  no  part  of  the  voluntary  musculature 
of  the  lateral  and  ventral  walls  of  the  neck  and  trunk  is  derived 
from  the  somatic  layer;  it  is  formed  entirely  from  the  myotomes 
which  gradually  extent,  ventrally  (Fig.  51)  and  finally  come  into 
contact  with  their  fellows  of  the  opposite  side  in  the  mid  ventral  line. 

m)  m-t  Will  it 

.mr 

Fio.  51.— DUGiAHS  Illostutimg  the  Histoxy  of  tbe  Gastxal  Mesodeih. 

itit  doxsal  portion  of  myotome;  gt,  genital  ridge;  /,  intestine;  Af ,  myotome,  Mr, 
membrana  retmiens;  iV,  nervous  system;  5C,  sderotome;  5w,  somatic  mesoderm; 
vm,  splanchnic  mesoderm;  vAf ,  ventral  portion  of  myotome;  Wi,  Wolffian  duct. 

Whether  the  voluntary  musculature  of  the  limbs  is  also  derived 
from  the  myotomes  is  at  present  doubtful.  It  has  been  very  generally 
believed  that  the  myotomes  in  their  growth  ventrally  sent  prolon- 
gations into  the  limb  buds  which  invested  the  axial  core  of  mesen- 
chyme and  eventually  gave  rise  to  the  voluntary  muscles.  The 
actual  existence  of  the  prolongations  of  the  myotomes  and  their 
conversion  into  the  limb  musculature  has,  however,  not  yet  been 
observed  and  it  is  qtiite  possible  that  the  limb  musculature  may  be 
derived  from  the  axial  core  of  somatic  mesoderm  from  which  the 
limb  skeleton  develops. 

The  appearance  of  the  mesodermic  somites  is  an  important 


THE   MESODEWJIC   SOMITES  g, 

phenomenon  in  the  development  of  the  embryo,  since  it  influences 
fundamentally  the  future  structure  of  the  organism.  If  each  pal 
of  mesodermic  som.tes  be  regarded  as  a  structural  unit  and  termed 
a»^«j..  or  segment,  then  it  may  be  said  that  the  body  is  com- 

us  fellows  and  succeedmg  one  another  at  regular  intervals.  Each 
~  differentiates,  as  has  been  stated,  into  a  sclerotome  and  a 
myotome,  and  accordmgly,  there  will  primarily  be  as  many  verte- 
b«  and  muscle  segments  as  there  are  mesodermic  somites,  or,  in 
other  words,  the  axial  skeleton  and  the  voluntaiy  muscles  of  the 
^k  are  primarily  metameric.  Nor  is  this  all.  Since  each 
metamere  IS  a  distmct  unit,  it  must  possess  its  own  supply  of  nutri- 
Uon,  and  hence  the  primary  arrangement  of  the  blood-vessels  is  also 
metamenc,  a  branch  passing  off  on  either  side  from  the  main  longi- 
tudmal  arteries  and  veins  to  each  metamere.  An  '  further,  each 
pair  of  muscle  segments  receives  its  own  ntrveS,  so  .t  the  aixange- 
ment  of  the  nerves,  again,  is  distinctly  met.imerc 

thP  H?»r  ^"T"^  '\"  '^'  metamerism  is  essentially  resident  in 
he  do.«il  mesoderm  the  segmentation  shown  by  structures  derived 
from  other  embryonic  tissues  V  .  secondary  and  associated  with 
the  relations  of  these  structure  the  .nesodermic  somites.  The 
metamerism  is  most  distinc.  in  tue  neck  and  trunk  regions,  and  at 
first  only  in  the  dorsal  portions  of  these  regions,  the  ventral  portions 
showing  metamerism  only  after  the  extension  into  them  of  the  myo- 

ZZ  T^  7  '"/^  ''"*'""  "''"  '^'  airangement  extends 
alsomtothehead,andthataportionofitsmesodermistoberegarded 
as  composed  of  metameres.  It  has  been  seen  that  in  the  noto- 
chordal  region  of  the  head  of  lower  ven.brates  mesodermic  somites 
are  present,  while  anteriorly  in  the  prechordal  region  there  are  head- 
cavities  which  resemble  closely  the  mesodermic  somites,  and  are 
probably  directly  comparable  to  the  somites  of  the  trunk  There  is 
reason  therefore,  for  believing  that  the  fundamental  arrangement 
of  the  dorsal  mesoderm  in  all  parts  of  the  body  is  metameric,  but 
hough  this  arrangement  is  cleariy  defined  in  early  embryos,  it 
loses  distmctness  in  later  periods  of  development.    But  even  in  the 


84 


IITERATUSE 


adult  the  original  metamerism  is  cleariy  indicated  in  the  arrange- 
ment of  the  nerves  and  of  parts  of  the  axial  skeleton,  and  careful 
study  frequently  reveals  indications  of  it  in  highly  modified  muscles 
and  blood-vessels. 

In  the  head  the  development  of  the  branchial  arches  and  clefts 
produces  a  series  of  parts  presenting  many  of  the  peculiarities  of 
metameres,  and,  indeed,  it  has  been  a  very  general  custom  to  regard 
them  as  expressions  of  the  general  metamerism  which  prevails 
throughout  the  body.  It  is  to  be  noted,  however,  that  they  are  pro- 
duced by  the  segmentation  of  the  ventral  mesoderm,  a  structure 
which  in  the  neck  and  trunk  regions  does  not  share  in  the  general 
metamerism,  and,  furthermore,  recent  observations  on  the  cranial 
nerves  seem  to  indicate  that  these  branchiomeres  cannot  be  regarded 
as  portions  of  the  head  metameres  or  even  as  structures  compara- 
ble to  these.  They  represent,  more  probably,  a  second  metamerism 
superposed  upon  the  more  general  one,  or,  indeed,  possibly  more 
primitive  than  it,  but  whose  relations  can  only  be  properly  under- 
stood in  connection  v/ith  a  study  of  the  c.-anial  nerves. 


LITERATURE. 

In  addition  to  many  of  the  papen  cited  in  tlie  list  at  ihe  close  of  Chapter  II,  the 
following  may  be  mentioned: 
C.  R.  Basdein:  "TheDevelopmentof  the  Musculature  of  the  Body  Wall  in  the  Pig, 

etc.,"  Ji^Hts  Hopkins  Hasp,  Rep.,  ix,  1900. 
T.  H.  Bkyce  and  J.  H.  Teacher:  "  Contributions  to  the  Study  of  the  Early  Develop- 
ment and  Imbedding  of  the  Human  Ovum,"  Glasgow,  xgoS. 
A.  C.  F.  Eternod:  "Communication  sur  un  oeuf  humain  avec  embryon   ezcesaive- 

ment  jetme,"  Arch.  Ital.  de  Biologie,  xxil,  1895. 
A.  C.  F.  Eternod:  "  II  y  a  un  canal  notochordal  dans  1' embryon  humain,"  Anal. 

Atueiger,  xvi,  1899. 
Fetzer:  "Ueber  ein  durch  Operation  gewonnenes  menschliches  Ei  das  in  seiner 

Entwickelung  ttwa  dem  Peterssehen  Ei  entspricht,"    Vtrh.  Anal.  GesettsckafI,  ■ 

XXIV,  1910. 
L.  Frassi:  "Weitere  Ergebnisse  des  Studiums  eines  jungen  menschlichen   Eies  in 

situ,"  Arch.f.  mikr.  Anat.,  Lxxi,  1908. 
W.  Hkape:  "The  Development  of  the  Mole  (Talpa  Europsea),"  Quarterly  Jam. 

Microsc.  Science,  xxvn,  1887. 
M.  Herzog:  "A  Contribution  to  our  Knowledge  of  the  Earliest  Known  Stages  of 

Placentation  and  Embryonic  Development  in  Man,"  Anur,  Jtmm.  Anat.t  IX,  X909. 


UTERATUM 


8S 


J- 


H.  Stkahl  and  R   Bevpitv-  "t^Ih  ■ 

J.  W.  VAN  W.Z:  "uTb?/die  MilT  ™""'^'^''"  Emb,y„,"  Wiesbaden.  .„o. 


CHAPTER  IV. 

THE  DEVELOPKEIfT  OF  THE  EXTERNAL  FORM  OF  THE 
HDMAN  EMBRYO. 

In  the  preceding  chapter  descriptions  have  been  given  of  human 
embryos  representing  the  earlier  known  stages  and  the  development 
of  the  general  form  of  the  human  embryo  has  been  traced  up  to  the 
time  when  the  mesodermic  .somites  have  made  their  appearance. 
It  will  now  be  convenient  to  continue  the  history  of  the  general 
development  up  to  the  stage  when  the  embryo  becomes  a  fetus. 

In  the  earlier  stages,  that  is  to  say  up  to  that  represented  by  the 
Etemod  embryo  (Fig.  43),  the  embryonic  disk  may  be  described  as 
floating  upon  the  surface  of  the  yolk-sac,  and  while  this  description 
still  holds  good  for  the  Etemod  embryo  a  distinct  groove  may  be  seen 
in  that  embryo  between  the  peripheral  portions  of  the  embryonic 
disk  and  the  upper  part  of  the  sac.  This  groove  marks  the  beginning 
of  the  separation  or  constriction  of  the  embryo  from  the  yolk-sac, 
the  result  of  which  is  the  transformation  of  the  discoidal  embryonic 
portion  of  the  embryonic  disk  into  a  cylindrical  structure.  Pri- 
marily this  depends  upon  the  deepening  of  the  furrow  which  sur- 
rounds the  embryonic  area,  the  edges  of  this  area  being  thus  bent  in 
on  all  sides  toward  the  yolk-sac.  This  bending  in  proceeds  most 
rapidly  at  the  anterior  end  of  the  body,  as  shown  in  the  diagrams 
(Fig.  52),  and  less  rapidly  at  the  posterior  end  where  the  belly- 
stalk  is  situated,  and  produces  a  constriction  of  the  yolk-sac,  the 
portion  of  this  structure  nearest  ihe  embryonic  disk  becoming  en- 
closed within  the  body  of  the  embryo  to  form  the  digestive  tract, 
while  the  remainder  is  converted  into  a  pedicle-like  portion,  the 
yolk-stalk,  at  the  extremity  of  which  is  the  yolk-vesicle.  The 
further  continuance  of  the  folding  in  of  the  edges  of  the  embryonic 
area  leads  to  an  almost  complete  closing  in  of  the  embryonic  coelom 

86 


DEVEIOPMENT  OF  EXTERNAt  FORM  g; 

and  reduces  the  opening  thro:igh  which  the  yolk-stalk  and  belly- 
stalk  communicate  with  the  embryonic  tissues  to  a  small  area  known 
as  the  umbilicus. 

In  the  Kramer  embryo  Klb  (Fig.  44)  this  separation  of  the  em- 
bryo proper  from  the  yolk-sac  has  proceeded  to  such  an  extent  that 
both  extremities  of  the  embryonic  disk  are  free  from  the  yolk-sac, 
and  the  anterior  extremity  is  beni  ventrally  almost  at  a  right  angle  to 


Fio.  sa.— DiAonAiis  IiLusxiuTmo  tei  Constiiction  op  iiii  Emiyo  fxoh  m 

Yolk-sac. 
AmdCim  longitudinal,  and  B  and  B  transverse  sections.    B  is  drawn  to  a  lane.  Kale 
than  the  other  figures.  ' 

the  rest  of  the  disk,  producing  what  is  termed  the  vertex  bend,  a 
feature  characteristic  of  all  later  embryos.  The  marked  develop- 
ment in  this  embryo  of  the  medullary  folds  and  the  occurrence  of 
mesodermic  somites  have  already  been  mentioned  (p.  72). 

Somewhat  more  advanced  is  the  BuUe  embryo  described  by 
Kollmann  and  shown  from  the  side  and  dorsally  in  Fig.  53,  the 
greater  part  of  the  yolk-sac  having  been  removed  as  well  as  the  most 
of  the  amnion.  The  embryo  measured  about  2.5  mm.  in  length  and 
showed  a  considerable  increase  in  the  number  of  mesodermic 
somites,  there  being  about  fourteen  of  them  on  either  side.    Pos- 


88 


DzvxtoniENT  or  bxtekmal  form 


teriorly  the  medullary  groove  has  become  converted  into  a  medul- 
lary canal  by  the  medullary  folds  meeting  over  it  and  fusing,  but 
anteriorly  it  is  still  open.    The  vertex  bend  is  well  marked  and 


«flj-)' 


.      .       „  ,  ..        Fio.  S3.— EiiBKYO  a.s  Hit.  Long. 
am,  Amnion;  B,  bdly^talkj  h,  heart;  U,  closed,  and  M',  stUl  open  portions  of  the 
medullary  groove;  Om,  vitelline  vein;  OS,  oral  fossa;  Y,  yolk-sac— (^oUmann.) 

immediately  behind  the  tip  of  the  head,  on  the  ventral  surface  of  the 
body,  there  may  be  seen  a  well-marked  depression,  the  oral  fossa, 
between  which  and  the  anterior  surface  of  the  yolk-sac  is  a  rounded 


DKVEL3PIIENT  OF   EXTERNAL   FORM 


89 


.  Fig.  54.— Embryo  Lr,  4.2  mm.  Long. 


90 


DEVELOPMENT  OF  BXTEBNAL  rOBM 


elevation  due  to  the  formation  of  the  heart.  Attention  may  be 
called  to  the  fact  that  the  position  of  this  organ  is  far  forward  of  that 
which  it  will  eventually  occupy,  so  that  it  must  undergo  a  marked 
retrogression  during  later  development. 

As  an  example  of  a  later  stage  of  development  the  embryo  £r  of 
His,  measuring  4.3  mm.  in  length,  may  be  taken  (Fig.  54).  In  this 
the  constriction  of  the  yolk-sac  has  progressed  so  far  that  its  proxi- 
mal portion  may  now  be  spoken  of  as  the  yolk-^lalk.  The  meso- 
dermic  somites  have  undergone  a  further  increase  and  have  almost 
reached  their  final  number,  the  vertex  bend  has  become  still  more 
pronounced  and  the  medullary  groove,  throughout  its  entire  length, 
has  been  converted  into  the  medullary  canal,  which,  anteriorly,  shows 
distinct  enlargements  and  M>n8trictions  which  foreshadow  various 
portions  of  the  future  brain.  The  auditory  organ,  which  made  its 
appearance  in  earlier  stages,  has  now  become  quite  distinct,  and  a 
lateral  bulging  of  the  most  anterior  portion  of  the  head  indicates  the 
position  of  the  future  eye. 

In  addition  certain  other  important  features  have  now  appeared. 
Thus,  about  opposite  the  head  a  second  bend,  the  nape  bend,  is 
becoming  visible  on  the  dorsal  surface  of  the  body  and  toward  the 
posterior  end  a  distinct  sacral  bend  is  evident  Secondly,  a  little 
posterior  to  the  level  of  the  nape  bend  a  slight  elevation  is  to  be  seen 
on  the  side  of  the  body;  this  is  the  limb  bud  for  the  upper  limb  and 
a  corresponding,  though  smaller,  elevation  in  the  region  of  the  sacral 
bend  represents  the  lower  limb. 

Thirdly,  three  grooves  having  a  dorso-ventral  direction  have 
appeared  on  the  sides  of  what  will  be  the  future  pharyngeal  region. 
These  are  representatives  of  a  series  of  branchial  clefts,  structures 
that  are  of  great  morphological  importance  in  the  further  develop- 
ment inasmuch  as  they  determine  to  a  large  extent  the  arrangement 
of  various  organs  of  the  head  region.  They  represent  the  clefts 
which  exist  in  the  walls  of  the  pharynx  in  fishes,  through  which 
water,  taken  in  at  the  mouth,  passes  to  the  exterior,  bathing  on  its 
way  the  gill  filamt  its  attached  to  the  bars  or  arches,  as  they  are 
termed,  which  separate  successive  clefts.    Hence  the  name  "bran- 


BEVBlOWttNT  OF  EXTERNAL  FOtM  ,, 

chial"  which  is  applied  to  them,  though  in  the  mammals  they  never 
ha«  respiratory  functions  to  perform,  but,  appearing   persist  for 
.  ..me  and  then  either  disappear  or  are  appliedTsome  enTe ly  i^f 
ferent  purpose.     Indeed,  in  man  they  are  never  really    lef  shut 
merely  grooves,  and  corresponding  to 
each  groove  in  the  ectoderm  there  ia 
also  one  in  the  subjacent  endoderm 
of  what  will  eventually  be  the  pharyn- 
geal region  of  the  digestive  tract,  so 
that  in  the  region  of  each  cleft  the 
ectoderm  and  endoderm  are  in  close 
relation,  being  separated  only  by  a 
very  thin  layer  of    mesoderm.     In 
the  intervals  between  successive  clefts 
a  more  considerable  amount  of  meso- 
derm is  present  (Fig.  55). 

In  the  human  embryo  four  clefts 
and   five  branchial  arches  develop 
on  each  side  of  the  body,  the  last  arch  lying  posteriorly  co  the  fourth 
cleft  and  not  bemg  very  sharply  defined  along  its  posterior  margin. 

As  just  stated,  the  clefts  are  normally  merely  grooves  and  in  !.»„ 
development  either  disappear  or  are  converted  inHPal  s,n^ctir« 
Occasionally,  however,  a  cleft  may  persist  and  the  thin  membraiJ^  wh^rh 

fe'"  h°°'  ""^  ^'""'  P*''°""'d  »  "«"  "«>  openingTrom  r«Terior 
mto  the  pharynx  occurs  at  the  side  of  the  neck,  foVminI  wh^  TtemitH  . 

^rZf^./'""'"}  """'i  ""  abnormality  is  n;ost  freqCSy  dlvXlS 
from  the  lower  (ventral)  part  of  the  first  cleft;  normaUy  diis  dSlnne^ 
Ae  upper  porUon  of  the  cleft  persisting,  however,  to  LTthe  exS 
auditory  meatus  and  tympanic  cavity.  external 

A  further  stage  in  the  differentiation  of  these  clefts  and  arches 
IS  shown  by  the  embryo  represented  In  Fig.  56.  The  nape  bend 
has  now  increased  to  such  an  extent  that  the  whole  anterior  part  of 
the  body  is  bent  at  a  right  angle  to  the  middle  part  and  the  entire 
embryo  is  coiled  in  a  spiral  manner.  The  limb  buds  are  much  more 
distinct  than  in  the  previous  stage  and  four  branchial  arches  are 
now  present;  the  second  and  third  have  become  more  defined  and 


0-  55;:-Floo«  or  tht  Pha>ynx 

or  EniYo  B,  7  itit.  Long. 
Ep,  Epiglottis;  5^,  .iniu  pnecmi- 
c'Im;    <',    tulxrculuin    imp«;    /t 

rJterKj-   portiom   o(   the  tongue! 
ir  III.  and  IV,  ^nlnlhial  aX. 
—(Km.) 


92 


DEVELOPMENT  OF  EXTEBNAL  FOKM 


a  Strong  process  has  developed  from  the  dorsal  part  of  the  anterior 
border  of  the  first  one,  which  has  thus  become  somewhat  •< -shaped. 
The  anterior  limb  of  each  V  is  destined  to  give  rise  to  the  upper  jaw, 
and  hence  is  known  as  the  maxillary  process,  while  the  posterior 
limb  represents  the  future  lower  jaw  and  is  termed  the  mandibular 
process. 


•      Fio.  56.-E101RYO  BXcKEK,  7.3  MM.  ra  LiMGTH.  X  s.-(Jr«»rf  o«rf  Hm.) 

In  the  stage  represented  by  this  embryo  the  closing  in  of  the 
embryonic  coelom  has  progressed  to  such  a  degree  that  only  a  small 
openmg  is  left  in  the  ventral  body-wall  of  the  embryo  through  which 
the  yolk-stalk  and  its  accompanying  vessels  and  the  belly-stalk  pass 
Indeed  the  margins  of  the  umbilicus  may  have  begun  to  be  pro- 
longed outward  over  these  structures,  enclosing  them  In  a  cylindrical 
investment,  the  first  stage  of  what  will  later  be  the  uvMical  cord 
being  thus  established. 


DEVELOPMENT  OF  EXTERNAL  rORlI  ^3 

His  and  m.  "V™™  Ruges  collection,  described  and  figured  bv 
H.S  and  measuring  9.  mm.  in  length*  tl,e  prolongatioll^of  the 


r'lO.  57.— ElIBlTO  9.1  MM.  LONO 

U,  LoTO  limb;  V.  umbilic<a  cord;  Vl,  upp„  limb;  Y,  yoUc.«c.-(ff.-,.) 

rr!L'llc°V''\""'''"'''"'  """'  '''"'"'"'  ""'"  -""^^  than  half  the 
yolk-stalk  has  become  enclosed  within  the  umbilical  cord     Th! 
nap.  and  sacral  bends  are  still  very  pronounced,  although  he  emb' 
s  begmnmg  to  straighten  out  and  is  not  quite  so  much  coM  asl 
the  preceding  stage.    At  the  posterior  end  of  the  body  there  his 


94 


DZVELOPUENT  OF  EXTERNAL  FOBll 


developed  a  rather  abruptly  conical  laUfilamml,  in  the  place  of  the 
blunt  and  gradually  tapering  termination  seen  in  earlier  stages, 
and  a  well-marked  rotundity  of  the  abdomen,  due  to  the  rapidly 
increasing  size  of  the  liver,  begins  to  become  evident. 

In  later  stages  the  enclosure  of  the  yollc-  and  belly-stalks  within 
the  umbilical  cord  proceeds  until  finally  the  cord  is  complete  through 
the  entire  interval  between  the  embryo  and  the  waU  of  the  ovum. 
At  the  same  time  the  straightening  out  of  the  embryo  continues,  as 
may  be  seen  in  Fig.  58  representing  the  embryo  xlv  (Br,)  of  His, 
which  shows  also,  both  in  front  of  and  behind  the  neck  bend,  a 


Fio.  58.— Embiyo  B  »„  13,6  UH.  Umo.~(Bis.) 

distinct  depression,  the  more  anterior  being  the  occipital  and  the  more 
posterior  the  nape  depression;  both  these  depressions  are  the  indica- 
tions of  changes  taking  place  in  the  central  nervous  system.  The 
tail  filament  has  become  more  marked,  and  in  the  head  region  a  slight 
ridge  surrounding  the  eyeball  and  marking  out  the  conjunctival  area 
has  appeared;  a  depression  anterior  to  the  nasal  fossa  marks  off  the 
nose  from  the  forehead;  and  the  external  ear,  whose  development 
will  be  considered  later  on,  has  become  quite  distinct.  This  embryo 
had  a  nape-rump  length  of  13.6  mm. 


DEVELOPMENT  OF   EXTEBNAl   PO«M  ^j 

His^otSrsSf  "^  ""'  '"'^  ^^'^  ^■■«-  59.  A  and  B)  of 
and  indeed  •:  ta^  f^t  I'h'  "'^  '^"^  "  P'°^-'^'"«' 
resemble  Cosety  the  fu'Cd  "rs'^^hTS.  ?''  •"«'"  '° 
what  reduced  in  size  still  tw.™:.,       i  .u  ^''  filament,  some- 

continues  to  be  wel  markT  Th  .'  '°'""'''^  °'  "'^  ^'«^<""- 
distinguishable  in    mb '„  1"  aL  r^°"  '"  '^'""''"'^  '°  »>« 

appeared  as  slight  .^^1^1':  ^^7,^-,-^  ^  S: 


oTtletsIrrertttiralSfe!^^^^^^ 
general  surface  of  the  face  wWkth!  '  t       ^    T'  '*"*^  ^•'°^«  ^« 
its  final  fetal  form.    S    Lb'f ^      " "^^ ''"°^' ''^''"^ 
IS  and  ,7.5  „un  in  lengS  respectively  about 

Pinollir      A»      — 1__ 

n  one  of  those  described 


•  ■n.e  embryo  S.  p««,te  .  sli 
"«.  but  othenra.  it  appears  to  be 


slight 


abnormalit 
normal. 


by  Hi», 

'y  >  Ihc-  great  projection  of  the 


96 


DEVKLOPHENT  OF  EXTERNAL  FORI! 


namely,  his  txxvn  (Wt),  having  a  length  of  23  mm.— may  be 
figured  (Fig.  60)  as  representing  the  practical  acquisition  of  the 
fetal  form.  This  embryo  .dates  from  about  the  end  of  the  second 
month  of  pregnancy,  and  from  this  period  onward  it  is  proper  to 
use  the  term  fetus  rather  than  that  of  embryo.    The  changes  which 


Fio.  60.— Ehbxyo  Wt,  33  HH.  Long.— (ffts.) 


have  been  described  in  preceding  stages  are  now  complete  and  it 
remains  only  to  be  mentioned  that  the  caudal  filament,  which  is  still 
prominent,  gradually  disappears  in  later  stages,  becoming,  as  it 
were,  submerged  and  concealed  beneath  adjacent  parts  by  the 
development  of  the  buttocks.  The  incompleteness  of  the  develop- 
ment of  these  regions  in  embryo  Wt  is  manifest,  not  only  from  the 


MVEtOPMENT  0,  THE  BBANCHIAt  A,CHES  ^j 

which  will  disappear  in  later  stages  ^°'  "  """^'''""^ 

four  branchial  c  Js  and  L  Irches  ThicTd^'T  '"  ^''^  ''•  '^^ 
embiyoare  visible  in  surface  viewfrutTnt?''  T  "'"""«" 
it  will  be  noticed  that  only  the  fi"; ':!  arches  T  7^°^' '«•  ''^ 
developed  ^a.illar,  process,  and  the^^sSarnrthr  ^jt 


Fio.  61.— Head  of  Embbvo  of  6  g  mm 
no.  Nasal  pit;  ps,  precervical  sinus  -(His  ) 

StSriJ'Si^:  -  sin.i„,  i„...d  Of  the  r^ion  occn- 

becon^e  the  anterior  p;rtfrlrreS     Th"   "'''"m  t"  '^'^^ 
an  embryo  (Br.)  described  hv  Hi      !   u     ^'''^ '» '>'ell  shown  in 


98 


DEVXLOPMXMT  Or  THE  BSANCHIAI.  AKCHZS 


of  the  second  arch  and  its  posterior  boundary  by  the  thoracic  wall, 
and  in  later  stages  these  two  boundaries  gradually  approach  one 
another  so  as  first  of  all  to  diminish  the  opening  into  the  sinus  and 
later  to  completely  obliterate  it  by  fusing  together,  the  sinus  thus 
becoming  converted  into  a  completely  closed  cavity  whose  floor  is 
formed  by  the  ectoderm  covering  the  three  posterior  arches  and  the 
clefts  separating  these.  This  cavity  eventually  undergoes  degen- 
eration, no  traces  oi  it  occurring  normally  in  the  adult,  although 


Fio.  61.— Faci  of  Euiyo  or  8  ini. 
»»#,  Maxillary  pnxm;  np,  uial  pit;  os,  oral  fosu;  pg,  proccssiu  globulaiis.— (His.) 

certain  cysts  occasionally  observed  in  the  sides  of  the  neck  may 
represent  persisting  portions  of  it. 

A  somewhat  similar  process  results  in  the  closure  of  the  ventral 
portion  of  the  first  cleft,*  a  fold  growing  backward  from  the  posterior 
edge  of  the  first  arch  and  fusing  with  the  ventral  part  of  the  anterior 
border  of  the  second  arch.  The  upper  part  of  the  cleft  persists, 
however,  and,  as  abeady  stated,  forms  the  external  auditory  meatus, 
the  pinna  of  the  ear  being  developed  from  the  adjacent  parts  of 
the  first  and  second  arches  (Figs.  58  and  59). 
*  S«e  page  91,  small  type. 


I 


BEVEtOPMBNT   OF   THE   FACB 

>s  already  us  Ir:':^'-  "^'^  '"'^'''P'-'"' 
'ength  (Fig.  6.)  .he  fossa  (o.)  has  asfum'o^  ''"""5  '  '"'"•  '" 
quadrilateral   form.    Its  p^terior   Z    f  "  '°'"'*'^'  ''^«"'" 

mandibukr  processes  fthnra^hrhn:T/\f™^  "^  ''"' 
by  the  maxillaiy  processes  (^^  J^^'    .    •    f  "'^"^  "  "  •^""'''^ 


Fig.  63.— Face  of  Embiyo  afte.  to.  n 

«V0  AFTE.  ra.  CoiromoN  OF  IBS  U,««  jAw.-(^i,.) 


;.^^r^rr:L^:^eXrt^'tX'-^— 

widely  separated,  the  nasal  pmceL  and  thTnf^  ^"""^T'  "' 

intervening  between  them,  aKey  art  i' "''T/''""""^ 

globttlarpHxess^byadee^andrftKeX^^^r^^^^^ 

opens  mto  a  circular  depression,  the  nasal  i^^p)     ^'"^  ""'^""''^ 


100 


DEVEloniENT  OF  THE  LIMBS 


Later  on  the  maxillary  and  globular  processes  unite,  obliterating 
the  groove  and  cutting  o«f  the  nasal  pits— which  have  by  this  time 
deepened  to  form  the  nasal  fossae— from  direct  communication 
trith  the  mouth,  with  which,  however,  they  later  make  new  com- 
munications behind  the  maxillary  processes,  an  indication  of  the 
anterior  and  posterior  nares  being  thus  produced. 

Occa^onaUy  the  maxillaiy  and  globular  processes  faU  to  unite  on  one 
or  l)oth  sides,  producing  a  condition  popularly  known  as  "harelip." 

At  the  time  when  this  fusion  occurs  the  nasal  fossae  are  widely 
separated  by  the  broad  nasal  process  (Fig.  63),  but  during  later 
development  this  process  narrows  to  form  the  nasal  septum  and  is 
graduaUy  elevated  above  the  general  surface  of  the  face  as  shown 
in  Figs.  58-^.  By  the  narrowing  of  the  nasal  process  the  globular 
processes  are  brought  nearer  together  and  form  the  portions  of  the 
upper  jaw  immediately  on  ^each  side  of  the  median  line,  the  rest 
of  the  jaw  being  formed  by  the  maxillary  processes.  In  the  mean- 
time a  furrow  has  appeared  upon  the  mandibular  process,  running 
parallel  with  its  borders  (Fig.  59);  the  portion  of  the  process  in  front 
of  this  furrow  gives  rise  to  the  lower  lip  and  is  known  as  the  lip 
ridge,  while  the  portion  behind  the  furrow  becomes  the  lower  jaw 
proper  and  is  termed  the  'A»»  ridge. 

The  DeTelopment  of  the  Limbs.— As  has  been  already  pointed 
out,  the  limbs  make  their  appearance  in  an  embryo  measuring  about 
4  mm.  in  length  (Fig.  54)  and  are  at  first  bud-like  in  form.  As  they 
increase  in  length  they  at  first  have  their  long  axes  directed  parallel 
to  the  longitudinal  axis  of  the  body  and  become  somewhat  flattened 
at  their  free  ends,  remaining  cylindrical  in  their  proximal  portions. 
A  furrow  or  constriction  appears  at  the  junction  of  the  flattened  and 
cylindrical  portions  (Fig.  57),  and  later  a  second  constriction  divides 
the  cylindrical  portion  into  a  proximal  and  distal  moiety,  the  three 
segments  of  each  limb— the  arm,  forearm,  and  hand  in  the  upper 
limb,  and  the  thigh,  leg,  and  foot  in  the  lower— being  thus  marked 
out.  The  digits  are  first  indicated  by  the  development  of  four 
radiating  shallow  grooves  upon  the  hand  and  foot  regions  (Fig.  58), 


DIVWOnONT  OF  THE  UUBS 

the  hand  or  of  the  Les  a^H  h^  "-e  d.gUal  and  palmar  region,  of 
-ched  .he  deve!:p::.\"^hX^lr'-  ^r  ""'  '"^^  " 
than  that  of  the  lower  althou^h^r  ^  °''"*'  """"'^  «?'% 

«^e  in  both  limbs.  The  Sf  K^n  r'*'"  ""^  "''""''"y  *« 
fi"t  to  a  very  considerable  ex,^t^".L°  "'"J*"'  '''^'"''y'  •""  ««  « 
further  growTh.  howeverdo^f!^  u"  '°«'""=^  *»  "  '^b,  whose 
these  thus  com  ng  tr^oi^]^!^''^  ^''  *'*  *»'  »'  *e  digit,, 
in  comparatively  early  Z«  1?^'  k""'  '"  '"'^^  """S"-  ^^en 
extent  the  great  Toe  swTddv  '^'""''' """^  *°  "  ^'"«'^»»«t  slighter 
(Figs-SPanfeo).      ■       "^"'^  '"P*"'"^''  f"""  'h*  second  digit 

havrSr:;*!?--^^^;;^-  -i^ing  P'r-'  «'>e  entire  limbs 
being  in  stages  later  tC  til  .^"'V"  '^'  "^^  "^  "^e  body, 
so  that  theirlngitudifaUx?' e  r  T  ^'''\  "  ""''''''  ^*»'™'5 
From  the  figures  of  ll"e  sta^s  t  m"'  K  """'" '°  "^'  °'  *<=  '^^J' 
(radial)  side  of  the  arm  2H  ^  ^  "^^  ^' ''  »  '''«'  '^umb 
which  are  directed  L™ a'd  S  T  '°'  ^'^''"^  '"■■^''  "^  '"«  '«« 
the  feet  and  han^  areTu^H  t  "^  !.T  *"''  P"'"^'  ^"^''^e^  ol 
directed  outward  and  Iwt  b?.  "/  "^^  ""'^  '"^  ^»--  « 
outwarf  and  slightly  foSd'  It  t  '  "'"'  ""=  '"^^  "^^ 
the  radial  side  oTthe  am  Somo  ^'u^''  '°  ^°°'^'"''«  «>«» 

1^.  the  palmar  surface^  he  W^T  ?*  ""=  "■'"■^'  ^'^e  of  the 
foot,  and  the  elbow  ^L  the  Le"    "  '''"'''  "*"  °'  "^^ 

longa^esagainbecomfparaSSZ^^^^^^^^^^^     "  *^'  "•- 
pushed  by  a  rotation  of  the  IT  i    '  '"^^-    ^'"'^  '^^ccom- 

shoulders  and  hip  W.  .t^  ^t^ •  ''T^  ''^°"«''  ">e 
tudinal  axes  through  an  ^n^leoT^f^:^^^^^^^^^^  '°"«"- 
the  upper  limb  is,  however,  in  ex^tlv^r  ^^'\""''  ^""^tion  of 
of  the  lower  limb  of  the  co^l^H  i  .  '^^""  '"'^"■''»  '°  '^at 
surfaces  of  the  two  limL  hSnSdv  d£'  "!''1  '•''^  '•°'»°'°«''- 
«aeofthearm.forins.nce,be?S:r:SS^;SS;^:^ 


I 

1 


loa  AGE  or  EMBRYO  AT  DimUMT  1TA0C8 

of  the  leg  is  the  inner  side,  and  whercM  the  palmar  surface  of  the 
hand  looks  ventrally,  the  plantar  surface  of  the  foot  looks  dorsalty. 
In  making  these  statements  no  account  is  taken  of  the  secondary 
position  which  the  hand  may  assume  as  the  result  of  its  pronation; 
the  positions  given  are  those  assumed  by  the  limbs  when  both  the 
bones  of  their  middle  segment  are  parallel  to  one  another. 

It  may  be  pointed  out  that  the  prevalent  use  of  the  physiological 
terms  flexor  and  extensor  to  describe  the  surfaces  of  the  limbs  has  a 
tendency  to  obscure  their  true  morphological  relationships.  Thus  If, 
as  is  usual,  the  dorsal  surface  of  the  arm  be  termed  its  extensor  surface, 
then  the  same  term  should  be  applied  to  the  entire  ventral  surface  of  the 
leg,  and  all  movements  of  the  loner  limb  ventrally  should  be  spoken  of  as 
movements  of  extension  and  any  movement  dorsally  as  movements  of 
flexion.  And  yet  a  ventral  movement  of  the  thigh  is  generally  spoken  of 
as  a  flexion  of  the  hip-joint,  while  a  straightening  out  of  the  foot  upon 
the  leg— that  is  to  say,  a  movement  of  it  dorsally— is  termed  its  extension. 

The  Age  of  the  Embryo  at  Different  Stages.— The  age  of  an 
embryo  must  be  dated  from  the  moment  of  fertilization  and  from 
what  has  been  said  in  preceding  pages  vpp.  27,  34)  it  is  evident  that 
it  must  be  difficult  to  determine  the  exact  date  of  this  event  from 
that  of  the  cessation  of  the  menses,  or  even  when  the  date  of  the 
coition  that  R"?ulted  in  pregnancy  is  known.  And,. furthermore, 
not  only  is  the  actual  date  of  the  beginning  of  development  uncertain, 
but  in  the  majority  of  known  early  human  embryos  the  time  of  the 
cessation  of  development  is  also  more  or  less  uncertain,  since  so 
many  of  these  embryos  are  abortions  and  their  expulsion  need  not 
necessarily  have  immediately  succeeded  their  death. 

These  various  sources  of  tmcertainty  are  of  especial  importance 
in  the  cases  of  embryos  in  the  early  stages  of  development,  when  a 
day  more  or  less  means  much,  and  it  seems  probable  that  many  of  the 
estimated  ages  given  for  young  embryos,  based  on  the  date  of  the 
last  menstruation,  are  too  low.  This  certainly  is  the  case  with  the 
ages  assigned  to  such  embryos  by  His,  who  estimated  embryos  of 
a.2  to  3.0  mm.  to  be  two  to  two  and  one-half  weeks  old,  those  of 
5.0  to  6.0  mm.  to  be  about  three  and  one-half  weeks  and  those  of 
lo.o  to  ii.o  mm.  to  be  about  four  and  one-half  weeks. 


■  3  "in.  In  loiiil. ,,,  Ik,  _.,C~V     .    ,'  'fT"  ""'  "mutin, 
—  d.,.  p»*»h  Lr,^  •<  ■  •«!•  »fc.  ™™„  „„„. 


I'fgOi  of  embi]'a 
In  nun. 


About    0.15 

'•3 
V.  B.  8.8 
V.  B.  14.0 
V.B.  aj.o 


D«y«  iniervening 

between  coition 

uid  abortion 


16) 
ax 

38 
47 
56 


ftebtble  age  in 
dtft 


13-14 
18-19 

37 
44-45 
53-54 


Authority 


Biytt  Teacher. 

Etemod 

Tandler. 

Rabl. 

MaU. 


I! 


I^  UTXIATUU 

limiti  of  error  (or  any  date  become  of  leu  importance.    Schrtder 
givet  the  following  meaiurementi  at  the  average: 

)cl  lunar  month 70-900101. 

4tli  lunar  nunili ioe-i;eimi. 

5ih  lunar  month 11)0-170  mm. 

6lh  lunar  month 180-340  mm. 

7lh  lunar  month 350-380  mm. 

8lh  lunar  month ^j  am. 

«th  lunar  month ^7  „„„_ 

loUi  lunar  month 490-joo  mm. 

The  data  concerning  the  weight  of  embryos  of  different  ages  are 
88  yet  very  insufficient,  and  it  is  well  known  that  the  weighta  of  new- 
bom  children  may  vary  greatly,  the  authenticated  extremes  being, 
according  to  Vierordt,  717  grams  and  6123  grams.  It  is  probable 
that  considerable  variations  in  weight  occur  also  during  fetal  life. 
So  far  as  embryos  of  the  first  two  months  are  concerned,  the  data  are 
100  imperfect  for  tabulation;  for  later  periods  Fehling  gives  the 
following  as  average  weights: 

jd  month jo  gtama. 

4th  month uo  gnmi. 

5th  month 185  grami. 

6th  month 63sgrmnu. 

7lh  month ,„o  grami. 

8th  month 1700  grams. 

9th  month „^o  gnuiu. 

10th  month 3,50  gramt 

and  the  results  obtained  by  Jackfon  are  essentially  similar. 

LITERATURE. 

In  addition  to  the  papers  of  Bryce  and  Teacher,  Etemod,  Fet«r,  Fiassi,  Henog, 
Peters,  Von  Spee  and  -Strahl  and  Beneke  cited  in  the  preceding  chapter,  the  tallowing 
may  be  mentioned: 

Buuu:  "Descriptbn  of  a  4  mm.  Human  Embryo,"  Amir.  Jam.  Anal.,  V,  1906. 
J.  BxoHAK:  "  Beobachtung  eines  menschlichen  Embryos  '  jn  beinahe  3  mm.  Linge 

mit  specieller  Bemerkung  Uber  die  bei  demselben    >efindlithen   Himralten," 

Morphotog.  Arbeitm,  v,  1895. 
A.  J.  P.  VAN  DEN  BaoEK:  "Zur  Kasuistilt  junger  menschlicher  Embrj-onen,"  Anat. 

Hefu,  xuv,  1911. 


UTIIATCai 

105 

A-  -»r.  -<«,."«x™^"«  """  ""^"^^  E^b^  vor^:.,  nu,.,.  Arc,. 

A.  Low:  "D«cripU„u  of  .  Human  EmbryoT.tl^M^^'  '**'• 

^<i«<.  niid  Phy,.,  xui  ,go8  ^    *  Mesodmnic  Somite,,"  Joun 

F.  P.  J^c:  "A  Human '..b^o  T.»„..,  b,,.  old,"  .-_.  „,  ^„^^^  ^ 

J   MH"«r^.7"?r  ^"""y"'^."  N-  York,  .8,,. 


io6 


UTESATUKX 


C.  Rabl:  "Die  Entwkklung  da  Golchto,  Heft  i,  Du  Geiicht  der  Slogetieie, 

Lelpdg,  1901. 
G.  RxrziDB:  "Zur  Kennhuss  der  Entwiddung   der  K5ipeifonnea  dea  Menschen 

wkhnnd  der  fsulen  Leboustiden,"  Bialag.  Vnlitsiuk.,  xi,  1904. 
J.  Tahdlu:  "Ueber  cinen  meiuchUchen  Embiyo  von  38  Tage,"  Anat.  Anuiitr, 

XXXI,  1907. 
Allen  Thohpson:  "Contributions  to  the  Hbtotjr  of  the  Structure  of  the  Rumin 

Ovum  and  Embryo  before  the  Third  Weclc  after  Conception,  with  a  Description 

of  Some  Early  Ova,"  Edinburgh  iltd.  and  Surg.  Journal,  m,  1839.     (See  also 

Froriep's  Neui  Ndiun,  xm,  1840.) 
P.  Thoufsoh:  "Description  of  a  human  embryo  of  twenty-three  paired  somites," 

Joum.  Anat.  and  Phys.,  XLI.  1907. 


CHAPTER  V. 

THE  YOLK-STALK,  BELLY-STALK,  AND  FETAL 
HEMBRAIIES. 

The  conditions  to  which  the  embryos  and  larvm  of  the  maioritv 

which  the  adult  organisms  exist  that  in  the  early  stages  of  devlt 
mm  special  organs  are  very  frequently  developed  which  are  of  u^ 
oriy  dunng  the  embryonic  or  larval  period  and  are  discarded  when 
r^!rW  T  '  u"''  "'  development  have  been  reached.  Tw" 
remark  apphes  w.th  especial  force  to  the  human  embryo  which  leads 

forapenod  of  nine  months  whatmay  be  termedapar^siticexistrce 
dn^mg  .U  nutrition  from  and  yielding  up  its  was'te  product  t  the' 
blood  of  the  parent.  In  order  that  this  may  be  accomplished  cer 
tern  special  organs  are  developed  by  the  emb^o,  by  means  ofthTh 
.t  forms  an  mtimate  connection  with  the  walUof  tke  utem  whi  h 
on  'ts  part,  becomes  greatly  modified,  the  combination  of  Tmb^o^t' 
and  maternal  structures  producing  what  are  termed  the  de^Z 

ll^SyX     '  "'  "■'*  '"^^  *"= ''-''''''  ""^-^ 

Furthermore  it  has  already  been  seen  that  many  peculiar  modi- 
fi«t.ons  of  development  in  the  human  embryo  result  f,^  the  inheri- 

Tll  "T  "^"^  ""'"  °^  '"^  "■"»'«  --tors,  and  amoTg 
Ae  embiyomc  adnexes  are  found  structures  which  represent  in  I 
mo«  or  less  modified  condition  organs  of  considerabfe  f^ctill 
.mportance  m  lower  forms.    Such  structures  are  the  yolk-stalk  Z 

T,'     "  "^T'  ^"'^  "^^  '^'^''  -<»  f-  'heir  pVpeTunde^ 

tandmg  U  w,U  be  weU  to  consider  briefly  their  developmem  in^me 

lower  form,  such  as  the  chick.  "h  m  some 

stri.^L'^ff  f"'  f '"J?"  '"'''^°  °^  *■"=  '''^''''^  ^^^  ">  be  con- 
stricted o£f  from  the  surface  of  the  large  yolk-mass,  a  fold,  consisting 

107 


j 
y   I 


io8 


YOIK-STAIK  AND   FETAl   MEMBRANES 


of  ectoderm  and  somatic  mesoderm,  arises  just  outside  the  embryonic 
area,  which  It  completely  surrounds.  As  development  proceeds  the 
fold  becomes  higher  and  its  edges  graduaUy  draw  nearer  together 
over  the  dorsal  surface  of  the  embryo  (Fig.  64,  A,  Af),  and  finally 
meet  and  fuse  (Fig.  64,  B  and  C),  so  that  the  embryo  becomes 
enclosed  within  a  sac,  which  is  termed  the  amnim  and  is  formed  by 
the  fusion  of  the  layers  which  constituted  the  inner  wall  of  the  fold 
The  layers  of  the  outer  wall  of  the  fold  after  fusion  form  part  of  the 


Fig.  64.-D1AORA1IS  Iilcstiiahno  ihi  Foumation  of  thi  Amnion  and  Auaktob 

IN  THE  ChiCE. 

Af,  Amnion  folds;  Al,  aUantois;  Am,  amniotic  cavity;  Ds,  yolk-Mc.—dSegaibaiir.) 

general  ectoderm  and  somatic  mesoderm  which  make  up  the  outer 
wall  of  the  ovum  and  together  are  known  as  the  serosa,  correspond- 
ing to  the  chorion  of  the  mammalian  embryo.  The  space  which 
occurs  between  the  amnion  and  the  serosa  is  a  portion  of  the  extra- 
embryonic coelom  and  is  continuous  with  the  embryonic  pleuro- 
peritoneal  cavity. 

In  the  ovum  of  the  chick,  as  in  that  of  the  reptile,  the  proto- 
plasmic material  is  limited  to  one  pok  and  rests  upon  the  large  yolk- 


THE  AHNION 


109 


absorbed  by  the  ImtZ'd^nlT         J''  '"""  ''"''«  «'^''"^"y 

size,  p.jecting  into  the  extra-emb 'ont  trtion  oHhe  n."'""  '•" 
toneal  cavity  and  pushing  before  itTi;;!.      i  "  P'europeri- 

lines  the  endoderm  (^H^  b°  nd  rt   t^"    "I'  ""''^^™  ^'''^^ 
reaching  a  very  ronsider.tl  ^^  u  ^"  "  '^'  '^""'"'  ^^ich, 

closely  fo  thel^sidT  fth^^^^^^^^^  ^'}^  ''PP'^-S  't-.f 

toiy  organ  for  the  emCn   f        ^T  ^^^P'^'oO'  and  excre- 

^^^.bi^S:^;-:--:---^^ 

aUan'^^^t^^runlet  tr'°"  ^^r  "-''  '''  ^'^^  ^^ 
the  hatching'^oVthe  "  ujch  kT^'r  '^''"«"'  ^""^  J"^'  »'^f°- 
and  closely' dberentTl1"',;'j  atT'  """•  '^'^  '^'^  "^ 
from  their  point  of  attach^enf to  tt'  hL      f^  '""'  ''P"^""« 


no  THE  AMNION 

fonns  are  greatly     .breviated  in  the  human  embryo.    The  envelop- 
ing layer,  instead  of  gradually  extending  from  one  pole  to  enclose 
the  entire  ovum,  develops  in  situ  during  the  stages  immediately 
succeeding   segmentation,   and   the    extra-embryonic   mescierm, 
instead  of  growing  out  from  the  embryo  to  enclose  the  yolk-sac, 
splits  off  directly  from  the  enveloping  layer.    The  earliest  stages  in 
the  development  of  the  amnion  are  not  yet  knf  rfn  for  the  human 
embryo,  but  from  the  condition  in  which  it  is  found  in  the  Peters 
embryo  (Fig.  37)  and  in  the  embryo  v.H.  of  von  Spee  (Fig.  39)  it 
is  probable  that  it  arises,  not  by  the  fusion  of  the  edges  of  a  fold,  as 
in  the  chick,  but  by  a  vacuolization  of  a  portion  of  the  inner  cell- 
mass,  as  has  been  described  as  occurring  in  the  bat  (p.  54).    It  is, 
then,  a  closed  cavity  from  the  veiy  beginning,  the  floor  of  the  cavity 
being  formed  by  the  embryonic  disk,  its  posterior  wall  by  the 
anterior  surface  of  the  belly-stalk,  while  its  roof  and  sides  are  thin 
and  composed  of  a  singly  layer  of  flattened  ectodermal  cells  lined 
on  the  outside  by  a  layer  of  mesoderm  continuous  with  the  somatic 
mesoderm  of  the  embryo  and  the  mesoderm  of  the  belly-stalk 
(Fig.  6s,  A). 

When  the  bending  downward  of  the  psripheral  portions  of  the 
embryonic  disk  to  close  in  the  ventral  surface  of  the  embryo  occurs, 
the  Une  of  attachment  of  the  amnion  to  tie  disk  is  also  carried 
yentraUy  (Fig.  65,  B),  so  that  when  the  constriction  off  of  the  embryo 
is  practically  completed,  the  amnion  is  attached  anteriorly  to  the 
margin  of  the  umbilicus  and  posteriorly  to  the  extremity  of  the  band 
of  ectoderm  lining  what  may  now  be  considered  the  posterior 
surface  of  the  beUy-stalk,  while  at  the  Jdes  it  is  attached  along  an 
oblique  line  joining  these  two  points  (Fig.  65,  B  and  C,  in  which  the 
attachment  of  the  amnion  is  indicated  by  the  broken  line). 

Leaving  aside  for  tfie  present  the  changes  which  occur  in  the 
attachment  of  the  amnion  to  the  embryo  (see  p.  116),  it  may  be 
said  that  during  the  later  growth  of  the  embryo  the  amniotic  cavity 
increases  in  size  until  finally  its  wall  comes  into  contact  with  the 
chorion,  the  extra-embryonic  body-cavity  being  thus  practically 
obliterated  (Fig.  65,  D),  though  lo  actual  fusion  of  amnion  and 


THE  AlfMION 


of  it  at  least  is  derived  from  the  embryo     It  is  a  fl„M    vu 

gravity  of  about  i  oo,  anrl  rnnt  ■\  ""*  '^'^  ^  specific 

principally  albumb    L"e,Zr°';^"'  '  P^  ^^»*-  "^  ^'Ms. 

probably  ^coming  "^orrrL,r\E  p.L^?  -^°■"''■'■-"' 
.uanfty-tbat  is  to  say,  at  about!  bS^fX  L^^S 


113  THE  YOLK-SAC 

of  pregnancy— it  varies  in  amount  between  one-half  and  three- 
fourths  of  a  liter,  but  during  the  last  month  it  diminishes  to  about 
half  that  quantity.  To  protect  the  epidermis  of  the  fetus  from 
maceration  during  its  prolonged  immersion  in  the  liquor  amnii,  the 
sebaceous  glands  of  the  skin  at  about  the  sixth  month  of  develop- 
ment pour  out  upon  the  surface  of  the  body  a  white  fatty  secretion 
known  as  the  vemix  caseosa. 

During  parturition  the  amnion,  as  a  rule,  ruptures  as  the  result 
of  the  contraction  of  the  uterine  walls  and  the  liquor  amnii  escapes 
as  the  "waters,"  a  phenomenon  which  normally  precedes  the 
delivery  of  the  child.  As  a  rule,  the  rupture  is  suflSciently  extensive 
to  allow  the  passage  of  the  child,  the  amnion  remaining  behind  in 
the  uterus,  to  be  subsequently  expelled  along  wit^  '"-  deciduffi. 

Occasionally  it  happens,  however,  that  the  amnion  is  sufBcienUy 
strong  to  withstand  the  pressure  exerted  upon  it  by  the  uterine  contractions 
and  the  child  is  born  sUU  enveloped  in  the  amnion,  which,  in  such  cases, 
IS  popularly  known  as  the  "caul,"  the  possession  of  which,  according  to 
an  old  superstition,  marks  the  child  as  a  favorite  of  fortune. 

As  stated  above,  the  liquor  amnii  varies  considerably  in  amount  in 
different  cases,  and  occasionsJly  it  may  be  present  in  excessive  quantities, 
producing  a  condition  known  as  hydramnios.  On  the  other  hand,  the 
amount  may  fall  considerably  below  the  normal,  in  which  case  the  anmii  n 
may  form  abnormal  unions  with  the  embryo,  sometimes  producing 
malformations.  Occasionally  also  bands  of  a  fibrous  character  traverse 
the  ammotic  cavity  and,  tightening  upon  the  embryo  during  its  growth, 
may  produce  various  malformations,  such  as  scars,  splitting  of  the  eyelids 
or  ups,  or  even  amputation  of  a  limb. 

The  Yolk-sac— The  probable  mode  of  development  of  the 
yolk-sac  in  the  human  embryo,  and  its  differentiation  into  yolk-stalk 
and  yolk-vesicle  have  abeady  been  described  (p.  86).  When  these 
changes  have  been  completed,  the  vesicle  is  a  small  pyriform  structure 
lying  between  the  amnion  and  the  chorionic  mesoderm,  some  dis- 
tance away  from  the  extremity  of  the  umbilical  cord  (Fig.  65,  D), 
and  the  stalk  is  a  long  slender  column  of  cells  extending  from  the 
vesicle  through  the  umbilical  cord  to  unite  with  the  intestinal 
tract  of  the  embryo.  The  vesicle  persists  until  birth  and  may  be 
found  among  the  decidual  tissues  as  a  small  sac  measuring  from  3  to 


WE  AIIANTOIS  AND  BELT-V-STAtK  „j 

10  mm.  in  its  longest  diameter.    The  stalt  J, 
goes  degeneration,  the  lumen  which  It  l^   r'''"'  ""'^  ""''*'- 
obliterated  and  its  endoden^  ,ll  ^  ^""^  """""^  '^°™m« 

of  the  second  monlh  oMTve^nfer'^TT'  "  "'^  "^  '"•=  '"^ 
which  extends  from  the  umbili  oH.t  '*""'"  "^  "^e  stalk 

the  degeneration  and  dLppe 'rbit  ^'"""'=  "^"""^  ^^ares  in 
persists,  forming  a  mo4  or  w  1^"' '"  '•?."'  ^  P"  «"'•  of  cases  it 
part  of  the  small  imestine  Zf  '  f  "'*"'-'"'"  of  the  lower 
length  and  sometinirrch  Urge  '"h  ra v'    '"  ""  "'^'^  "^  ^^  "■ 

Ae  occurrence  of  the  diverticulum  mavh?-^^'^i',''*PP'^"^  '«'»tively  to 

b.o<5-:;S:rat''^:S^^^^^  '^  ^•'-'^-'X  suppled  With 
on.  and  even  although  theWil;  f"^''""  °'  "*'  y°"'  '^  "^^^^d 
an  organ  of  nutn'tion  s  a  2:^'^^""  "'  ''^  ^""'"-^  ^ 
still  retains  a  well-develo^^dwlo^'  f*'  ^°'""  ^"^ryo,  yet  it 
esi^ially  possessing  a  Hcrnetlr^oTSr  ''rt  f  I''  7'''" 

The  AUantois  and  Belly-stalk —Tf  k  u 
reptilian  and  avian  embryos  the  anfntn,  I  ''°  '"""  """t  '" 
development  and  functionTas^reStoLTnd  "  '''''  "^'^  °' 
coming  into  contact  with  what  is  comn  \,  ^  ""*'°^  °'-«^"  by 
mammalian  embryo  I„  Zn  '^.^  \'°  '^'  =''°"°°  of  the 
mode  of  development  and'n ".  '  ,a7'  """'  u™^''*''  '"''••  '"  ■''' 
resemblance  to  the  avian  orlan  I  '"  ""^'^  P"'='  ^o  that  its 


"4 


1^  AUANTOIS  AND  pKUY-STAtK 


maternal  blood,  instead  of  with  the  external  atmosphere,  as  is  the 
case  in  the  egg-laying  forms.  Thus,  the  endodermal  portion  of  the 
allantois,  instead  of  arising  from  the  intestine  and  pushing  before 
it  a  layer  of  splanchnic  mesoderm  to  form  a  large  sac  lying  freely  in 
the  extra-embryonic  portion  of  the  body-cavity,  appears  in  the  human 
embryo  before  the  intestine  has  differentiated  from  the  yolk-sac  and 
pushes  its  way  into  the  solid  mass  of  mesoderm  which  forms  the 
belly-stalk  (Fig.  6$,  A) .  To  understand  the  significance  of  this  proc- 
ess it  is  necessary  to  recall  the  abbreviation  in  the  human  embryo  of 
the  development  of  the  extra-embryonic  mesoderm  and  body-cavity. 
Instead  of  growing  out  from  the  embryonic  area,  as  it  does  in  the 
lower  forms,  this  mesoderm  develops  *»  situ  by  splitting  off  from 
the  layer  of  enveloping  cells  and,  furthermore,  the  extra-embryonic 
body<avity  arises  by  a  splitting  of  the 
mesoderm  so  formed  before  there  is  any 
trace  of  a  splitting  of  the  embryonic 
mesoderm  (Fig.  38).  The  belly-stalk, 
whose  development  from  a  portion  of 
the  inner  cell-mass  has  already  been 
traced  (p.  68),  is  to  be  regarded  as  a 
portion  of  the  body  ot  the  embrvo, 
since  the  ectoderm  which  covers  one 
surface  of  it  resembles  exactly  that  of 
the  embryonic  disk  and  shows  an  ex- 
tension backward  of  the  medullary 
groove  upon  its  surface  (Fig.  66).  The 
mesoderm,  therefore,  of  the  belly-stalk 
"s  to  be  regarded  as  a  portion  of  the  embryonic  mesoderm  which  has 
not  yet  undergone  a  splitting  into  somatic  and  splanchnic  layers, 
and,  indeed,  it  never  does  undergo  such  a  splitting,  so  that  there  is 
no  body-cavity  into  which  the  endodermal  allantoic  diverticulum 
can  grow. 

But  this  does  not  account  for  all  the  peculiarities  of  the  human 
allantois.  In  the  birds,  and  indeed  in  the  lower  oviparous  mammals, 
the  endodermal  portion  of  the  allantois  is  equally  developed  with 


Fro.  66.— TuANsvMai  Sic- 
noN  THKOuca  thz  Belly-stalk 

or  AN  EUBEYO  OP  3.15  HH. 

Aa,  Umbilical  (ilUntoic) 
arteijp;  All,  mllantois;  am,  am- 
nion; Fa,  umbilical  (allantoic) 
vein. — (His.) 


I 


tHE  ALIANTOIS  AND   BEHY-STAtl  „, 

ct:it"rj,rXTd  it"'"-  ^""^ "-  ---  -  "hose 

lying  in  the  substonr!  „f  fv.^       .       '""''"'"'= '°  ">«  =h°rion  and 
with  the  relatively  thin  i  t     f  ^garded  as  homologous 

endc^er  JXtVcl^:;  It^k'-'T^r  ^°^'""«  ''''= 
disparity  in  the  develooment  lAl,  .      "Planafon  of  this 

portions  of  the  humrnarnT-  '\'"'^"'^^  and  endodermal 
conditions  under  Xh  2  "  w  '"'  '"  "^  '""""^  '^  t'''^  <"'"«! 
Inallforn>s,the  ole  fswelaXTh  '"?'*.  ^--"■°"  '-"e  place, 
is  the  more  importarc:ient%' ^  ^^^^^^^^^ 
blood-vessels,  upon  whose  n,»..l  .7  aUantois,  smce  m  it  the 
depend,  arise  ^7  j'ZST^''-;:^^^^^  -'""="°"^ 
mammals  there  are  n«  ™„      u       , .  ^  °"^^^  ^"'*  oviparous 

passed  to  the  Lerior  of Te  n  '  "^'.'  ""'"^•'  "^'"'^l  «"  be 
within  the  cav'Hf  the  ^,  7"'  T'^  ''  "'  "'"^^°^^>-  ^'^^d  uP 
considerableTuan.il  on^'°'^'''  ""^'"'''^  fl^id  ""taining 
In  the  higher  mail  theint  T^  '","^''°«  ""^  P^^"^'^  »'  -^a 
the  chorfon  andTteri^S  if ''''T:''''''>'^'=^<='°P  »>-'-- 
fluids  into  the  ma  ema  bl^  aS  S"  °'  *'  P"^*'  "'  «"«'"• 
the  less  necessity  thT  is  for'.n  l^  .""'""'^  *'''^"«'^"<»«. 
fluid  may  be  stored  up     ThZ^  "■  '"^'^  '"  ^'^«''  =«^«ed 

cavity  in^he  nTii.  fl^e^'^^^raVd  1^  r'T^"'  "'  '"^ 
upon  the  greater  intimacy  of  thel^ L  bet^      "'"'''  P'"''"'''' 

nthektter.thearrangemLtfor  hrZatrffh  """"  "f  "'^""^ 
into  the  maternal  blood  being  si  pXTt  .  '■'"''"'^  "'"^"'^ 
need  for  the  development  of  an"  lES  itltf "  '^  ^"'^'^^"^  °° 

Ihe  portion  of  the  endodermal  diverticulnm  „.,•  u  ■ 
wuhin  the  umbilical  cord  pers^    un^  bS,  "  '°''°^ 

rudimentary  condition   but  thp  In!  """''""*'•  '«  »  more  or  less 

f-  the^ex  of  trie?  o  r:S2r""'"  ^^'="'^-"« 

■nto  a  solid  cord  of  fibrous  tissue  terj^^he  ::i!'""  "°""^ 


ii6 


THE   UMBIUCAI.  COkD 


Occaitonally  •  lumen  penists  in  the  urachal  portion  of  the  aUantoii 
and  mar  open  to  the  exterior  at  the  utnbiliau,  in  which  caM  urine  from 
the  bladder  may  eKape  at  the  umbilicus. 

Since  the  allantois  in  the  human  embryo,  as  well  as  in  the  lower 
forms,  is  responsible  for  respiration  and  excretion,  its  blood-vessels 
are  well  developed.  They  are  represented  in  the  belly-stalk  by 
two  veins  and  two  arteries  (Fig.  66),  known  in  human  embryology 
as  the  umbilical  veins  and  arteries.  These  extend  from  the  body  of 
the  embryo  out  to  the  chorion,  there  branching  repeatedly  to  enter 
the  numerous  chorionic  villi  by  which  the  embryonic  tissues  are 
placed  in  relation  with  the  maternal. 

The  Umbilical  Cord.— During  the  process  of  closing  in  of  the 
ventral  surface  of  the  embryo  a  stage  is  reached  in  which  the  em- 
bryonic and  extra-embryonic  portions  of  the  body-cavity  are 
completely  separated  except  for  a  small  area,  the  umbilicus,  through 
which  the  yolk-stalk  passes  out  (Fig.  65,  B).  At  the  edges  of  this 
area  in  front  and  at  the  sides  the  embryonic  ectoderm  and  somatic 
mesoderm  become  continuous  with  the  corresponding  layers  of  the 
amnion,  but  posteriorly  the  line  of  attachment  of  the  amnion  passes 
up  upon  the  sides  of  the  belly-stalk  (Fig.  6$,  B),  so  that  the  whole  of 
the  ventral  surface  of  the  stalk  is  entirely  imcovered  by  ectoderm, 
this  layer  being  limited  to  its  dorsal  surface  (Fig.  66).  In  sub- 
sequent stages  the  embryonic  ectoderm  and  somatic  mesoderm  at 
the  edges  of  the  umbilicus  grow  out  ventrally,  carrying  with  them 
the  line  of  attachment  of  the  amnion  and  forming  a  tube  which 
encloses  the  proximal  part  of  the  yolk-stalk.  The  ectoderm  of  the 
belly-stalk  at  the  same  time  extending  more  laterally,  the  condition 
represented  in  Fig.  65,  C,  is  produced,  and,  these  processes  con- 
tinuing, the  entire  belly-stalk,  together  with  the  yolk-stalk,  becomes 
enclosed  within  a  cylindrical  cord  extending  from  the  ventral 
surface  of  the  body  to  the  chorion  and  forming  the  umbilical  cord 
(Fig.  65,  D). 

From  this  mode  of  development  it  is  evident  that  the  cord  is, 
strictly  speaking,  a  portion  of  the  embryo,  its  surfaces  being  com- 
pletely covered  by  embryonic  ectoderm,  the  amnion  being  carried 


rai  CMBttlCAL  COlO 


Mr 


UV 


ii8 


THX  CHOUON 


during  Itt  formation  further  and  further  from  the  umbilicus  ib.'.i1 
finally  it  it  attached  around  the  distal  extremity  of  the  cord. 

In  enclosing  the  yolk-stalk  the  umbilical  cord  encloses  also  a 
small  portion  of  what  was  originally  th^  extra-embryonic  body- 
cavity  surrounding  the  yolk-stalk.  A  section  of  the  cord  in  an  early 
stage  of  its  development  (Fig.  67,  A)  will  show  a  thick  mass  of 
mesoderm  occupying  its  dorsal  region;  this  represents  the  mesoderm 
of  the  belly-stalk  and  contains  the  allantois  and  the  umbilical 
arteries  and  vein  (the  two  veins  originally  present  in  the  belly-stalk 
having  fused),  while  toward  the  ventral  surface  there  will  be  seen  a 
distinct  cavity  in  which  lies  the  yolk-stalk  with  its  accompanying 
blood-vessels.  The  portion  of  this  coelom  nearest  the  body  of  the 
embryo  becomes  much  enlarged,  and  during  the  second  month  of 
development  contains  some  coils  of  the  small  intestine,  but  later  the 
entire  cavity  becomes  more  and  more  encroached  upon  by  the 
growth  of  the  mesoderm,  and  at  about  the  fourth  month  is  entirely 
obliterated.  A  section  of  the  cord  subsequent  to  that  period  of 
development  will  show  a  solid  mass  of  mesoderm  in  which  are 
embedded  the  umbilical  arteries  and  vein,  the  allantois,  and  the 
rudiments  of  the  yolk-stalk  (Fig.  67,  B). 

When  fully  formed,  the  umbilical  cord  measures  on  the  average 
SS  cm.  in  length,  though  it  varies  considerably  in  different  cases,  and 
has  a  diameter  of  about  1.5  cm.  It  presents  the  appearance  of  being 
spirally  twisted,  an  appearance  largely  due,  however,  to  the  spiral 
course  pursued  by  the  umbilical  arteries,  though  the  entire  cord  may 
undergo  a  certain  amoimt  of  torsion  fiuji  the  movements  of  the 
embryo  in  the  later  stages  of  development  and  may  even  be  knotttd. 
The  greater  part  of  its  substance  is  formed  by  the  mesoderm,  the 
cells  of  which  become  stellate  and  form  a  recticulum,  the  meshes 
of  which  are  occupied  by  connective-tissue  fibrils  and  a  mucous  fiuid 
which  gives  to  the  tissue  a  jelly-like  consistence,  whence  it  has  re- 
ceived the  name  of  Wharton's  jelly. 

The  Chorion. — To  understand  the  developmental  changes 
which  the  chorion  undergoes  it  will  be  of  advantage  to  obtain  some 
insight  into  the  manner  in  which  the  ovum  becomes  implanted  in 


nx  CHOUON 


"9 


F.0.68.-S„CCESS™StAOESMIH,lM,^„,^«„^0 

«,  syncytial  k™>b;  *,  ■„„„  a»-m.^(^^^^""'°'^*- 
Of  the  central  cells  is  almost  completed  (Fig  68  A^     At 
of  he  covering  layer  the  cells  beco-nethtker  andtt^f  '''"°" 

cytial  projection  or  knob  which  comes  into  contact  Jhhr  1  '^°' 
mucosa  (Fig.  68,  B),  and  at  the  point  of  contact  tht,,  "T 
undergo  degeneration,  allowing  thrknoh  ,  ■  """^  ""* 

the  deeper  Lues  of  the  utZ  ^.^f^.  7"°  ^^'«"--  with 
being  one  in  wWch  the  mucosa  ce  Islre  eiSed' j!  T''""  'P^f  ['""' 
It  seems  probable  that  in  the  humin  o1^"'h foT     '       ':°''' 
of  a  Similar  nature  and  thatas  the  cove^^li^er^Xr  i^ 


THE  CHOBION 


FlO.  69. — DUOXAIIS  lUUSTIATWO  THE  IHPUNTATION  OF  im  OVUlf. 

ttc,  amniotic  cavity;  hs,  bdly-stalk;  c/,  chorion  frondosum;  ci,  chorion  lzve;<fe, 
decidua  capaularis;  fc,  inner  cell-mass;  »,  space  surrounding  ovum  wh"-n  becomes  the 
intervilloua  space;  Mm,  uterine  mucosa;  v,  chorionic  villus;  ys,  yolk-sa< . 


THE   CHORION  jj^ 

contact  With  the  deeper  layers  Of  the  uterus,  these  too  are  eroded  and 
the  utenne  blood-vessels  being  included  in  the  eroln^t's  an 
ext^vasafon  of  blood  pUsma  and  corpuscles  occu"   "  fhTvTcL  y 

raoid  anH  L  .     ^  '"^^  '"""'y  ''«8es  being  especially 

.1 «».  „  „  „„ .  „„„,„  „„  y,j  ^  ^j  ^^j  ^^^^^  j^ 


—  B.U. 


Fio.  70.— Section  of  an  Ovum  of  i  mm     a  c.^  „ 

pletely  enclose  it,  forming  an  envelope  known  as  the  A.  ^ 
sularn  or  refle^a.    The  blood  extra vaLionirnow  Itat^"' .?" 
a  closed  space  bounded  on  the  one  hand  by  he  uteTnf  ,^  '"""" 
on  the  other  by  the  wall  of  the  ovum  (Fig  69  B)  '"*"  ""'' 

The  youngest  known  human  ova  have  abeady  reached  app«,xi- 


"'  THE  CHOSION 

mately  this  stage.  Thus,  the  Peters  ovum  (Fig.  70)  had  already 
sunk  deeply  into  the  uterine  mucosa,  the  point  of  entrance  being 
indicated  by  a  gap  in  the  decidua  capsularis,  closed  in  this  case  by  a 
patch  of  coagulated  blood  (Sck).  The  uterine  tissues  in  the  imme- 
diate vicinity  of  the  ovum  were  much  swollen  and  apparently  some- 
what necrotic  and  their  blood-vessels  could  be  seen  to  communicate 
with  the  space  between  the  wall  of  the  ovum  and  the  maternal  tissues. 
This  space,  however,  was  converted  into  an  irregular  network  of 
blood  lacuna  by  anastomosing  cords  of  cells,  which  arose  from  the 
wall  of  the  ovum  and  extended  through  the  space  to  the  maternal 
tissues;  these  cords  of  cells  are  represented  in  Fig.  70  by  the  darker 
masses  projecting  from  the  wall  of  the  ovum  and  scattered  among 
the  paler  blood  lacunae.  This  stage  of  implantation  of  the  ovum  is 
shown  diagrammaticaUy  in  Fig.  69,  B,  where,  for  simplicity's  sake, 
the  cell  cords  are  represented  merely  as  processes  radiating  from 
the  ovum  without  reaching  the  maternal  tissues. 

The  cell  cords  are  derivatives  of  the  trophoblast  and  are,  there- 
fore, of  embryonic  origin.  If  examined  under  a  higher  magnifica- 
tion than  that  shown  in  Fig.  70  they  v  iU  be  seen  to  be  composed  of  an 
axial  core  of  cells  with  distinct  outlines,  enclosed  within  a  layer  of 
protoplasm  which  lacks  all  traces  of  ceU  boundaries,  ahhough  it 
contains  numerous  nuclei,  being  what  is  termed  a  syncytium  or 
Plasmodium.  The  original  trophoblast  has  thus  become  differen- 
tiated into  two  distinct  tissues,  a  cellular  one,  which  has  been  termed 
the  cyio-trophoblast,  and  a  plasmodial  one,  which,  similarly,  is 
known  as  the  plasmodi-trophoblast  and  is  the  tissue  that  comes  into 
contact  with  the  maternal  blood  contained  in  the  lacunar  spaces  and 
with  the  maternal  tissues,  in  connection  with  these  latter  sometimes 
developing  into  masses  of  considerable  extent.  To  this  plasmodi- 
trophoblast  may  be  ascribed  the  active  part  in  the  destruction  of 
the  maternal  tissues  and  probably  also  the  absorption  of  the  products 
of  the  destruction  for  the  nutrition  of  the  growing  ovum.  For  up  to 
this  stage  the  ovum  has  been  playing  the  rdle  of  a  parasite  thriving 
upon  the  tissues  of_its  host. 

The  food  material  that  the  ovum  thus  obtains  may  conveniently 


THE  CHOSION 


133 


up  tTht'ttrandr^*'"'-''^  '^°'  P^"'"'''-  -Wch  obtains 
another2Sha!blrn     in      r    "'"  "^"^  ""=  P^P""""-  '- 

t«.phobi:sSutt^ws  o^ri"  ?r  "^ ""  ^^'^  '^•^^'^  ^"'■■^•^'^ 

mesoderm  mlv  bV  J^n  ^  ^'  ^°^  P'""""'"'  f™"  "^=  ^l^orionic 

axis  of  ichtc^d  the  In  T'''  "'""*  ''"'^'  '"•'»  ^''«'>- '"'°  "^ 

cords  tHete^^bt:rr:rLfo\r-'''^^^"^"'  "-^ 

vuuvenea  into  branding  processes,  the 


Fio. 


71- 


-Entim;    Ovum   Aboeted  at  about  t».  c 

Month.    X^T-'cS^'™"""'  °"==  S^co™ 

chorionic  villi,  which  protect  from  (he  .„.:,       -^ 
(H«.  ;.)  into  the  -oLL« l;",^\S'l ^^^^^^^^^^ 
.«<.m//^  ./>„«,  and  are  bathed  by  the  materTl  mI^     u-     ^' 
contains.     Toward  the  maternal  surfL  of  rhesTa^  ^^^j'"^''  '! 
the  ti«phoblast  still  persist,  uniting  the  extremitl.!.;  '''' °^ 

villi  to  the  enclosing  uterine  wall  sucV^h  K  ""^'°  °^  *•= 

viUi  to  distinguish  them  from  the  LSy  1  ?'  ''""'''  ^""^ 
the  intervillous  space.    Later.  12^ '  el^S^l^l^i 


184 


THE   CHOBION 


develop,  those  associated  with  the  allantois  extend  outwaid  into 
the  chorionic  mesoderm  and  thence  send  branches  into  each  villus. 
The  second  type  of  placentation,  the  hcemotrophic  type,  is  thus  estab- 
lished, the  fetal  blood  contained  in  the  vessels  of  the  villi  receiving 
nutrition  through  the  walls  of  the  villi  from  the  maternal  blood 
contained  in  the  intervillous  space,  and,  similarly,  transferring 
waste  products  to  it. 

At  first,  as  stated  above,  the  villi  usually  cover  the  entire  surface 
of  the  ovum,  but  later,  as  the  ovum  increases  in  size,  those  villi 
which  are  remote  from  the  attachment  of  the  belly-stalk  to  the  chorion 
are  placed  at  a  disadvantage  so  far  as  their  blood  supply  is  concerned 


Fig.  73.— Two  Villi  rRoii  the  Chorio.n  of  an  Embeyo  or  7  ui. 

and  graduaUy  disappear,  and  this  process  continues  until,  finally, 
only  those  villi  are  retained  which  are  in  the  immediate  region  of 
ihe  belly-stalk  (Fig.  69,  C),  these  persisting  to  form  the  fetal  portion 
of  the  placenta.  By  these  changes  the  chorion  becomes  diSFerenti- 
ated  into  two  regions  (Fig.  69,  C),  one  of  which  is  desi!tutc  of  viUi  and 
is  termed  the  chorion  lave,  while  the  other  provided  with  them,  is 
known  as  the  chorion  frondosum. 


THE  CHORION 


MS 


136 


THE   CHOUON 


Occanonally  one  or  more  patches  of  villi  may  persist  in  the  area  that 
normaUy  becomes  the  chorion  laeve  and  thus  accessory  placenta  {placenta 
succentwtata),  varying  in  number  and  size,  may  be  formed. 

The  villi  when  fully  formed  are  processes  of  the  chorion,  branch- 
ing profusely  and  irregularly  (Fig.  72),  and  each  consists  of  a  core  of 
mesoderm,  containing  blood-vessels,  enclosed  within  a  double 
layer  of  trophoblastic  tissue  (Fig.  73,  A).  The  inner  layer  consists 
of  a  sheet  of  well  defined  cells  arranged  in  a  single  series;  it  is 
derived  from  the  cyto-trophoblast  and  forms  what  is  known  as  the 
layer  of  Langhans  cells.  The  outer  layer  is  syncytial  in  structure 
and  is  formed  from  the  plasmodi-trophoblast. 


<Ti 


Fio.  74.— Matdse  Placenta  after  Sepaeatiok  raoK  the  Uteius. 
c,  Cotyledons:  ch,  chorion,  amnion,  and  decidua  vera;  urn,  umbilical  coid.— (JCottuKnm.) 

As  development  proceeds  the  villi,  which  are  at  first  distributed 
evenly  over  the  chorion  frondosum,  become  separated  into  groups 
termed  cotyledons  (Fig.  74)  by  the  growth  into  the  interviUous  space 
of  trabecute  from  the  walls  of  the  uterus,  the  fixation  villi  becoming 
connected  with  these  septa  as  well  as  with  the  general  uterine  wall. 
The  ectoderm  of  the  villi  also  undergoes  certain  changes  with  ad- 
vancing growth,  the  layer  of  Langhans  cells  disappearing  except  in 
small  areas  scattered  irregularly  in  the  villi,  and  the  syncytium, 


TBE  CSOUON 


laj 


Sd.^"::^^TiT:„'-/    t^enings   .Mch    become 
(Fi«-73,B,cA  "'*»^'vely,   by   depositions    of    fibrin 

in  tle^tnT a;'' vtri?T  ''^  ""'"  '""^^^  °'  '"=-'°P-nt 
are  very  similar  to  those  described  for  the  villi. 


no.  75.— Section  through  ihi  Placentai  -r^™ 

,  „  „  ,  m"^.  '*'""°''  °'  *»  E-B.VO  OE  Seven 

Thus,    the   mesoderm    thickeni    Jto   «, . 


taS 


THE  DBClDUJt 


in  irregular  patches  by  a  peculiar  form  of  fibrin  which  is  traversed 
by  flattened  anastomosing  spaces  and  to  which  the  name  canalized 
fibrin  or  fibrinoid  has  been  applied  (Fig.  75). 

The  Decidu«.— It  has  been  pointed  out  (p.  26)  that  in  connec- 
tion with  the  phenomenon  of  menstruation  periodic  alterations 
occur  in  the  mucous  membrane  of  the  uterus.  If  during  one  of 
these  periods  a  fertilized  ovum  reaches  the  uterus,  the  desquamation 


Fio.  76.-DUOHAH  SHOwmo  the  Rilaiions  of  ra«  Fetal  Mejoeanes. 

Am,  Amnion;  C»   chorion;  AfmiBcular  wall  of  uterus;  C,  decidua  capsularis;  B. 

decidua  basalLs;  V,  decidua  vera;  y,  yolk-sUlk. 

of  portions  of  the  epithelium  does  not  occur  nor  is  there  any  appre- 
ciable hemorrhage  into  the  cavity  of  the  uterus;  the  uterine  mucosa 
remains  in  what  is  practically  the  ante-menstrual  condition  until  the 
conclusion  of  pregnancy,  when,  after  the  birth  of  the  fetus,  a  con- 
siderable portion  of  its  thickness  is  expeUed  from  the  uterus,  forming 
what  is  termed  the  decidua:.    In  other  words,  the  sloughing  of  the 


THE  DrciDDJt 


t»9 


uterme  tissue  wh.ch  concludes  the  process  of  menstruation  is  post- 
poned unt.I  the  close  of  pregnancy,  and  then  takes  place  simultane- 
ously over  the  whole  extent  of  the  uterus.  Of  course,  the  changes 
n  the  uterme  tissues  ar  «,mewhat  more  extensive  during  pregnancy 
SurT"*  "jenfuation,  but  there  is  an  undoubted  fundamental 
similarity  in  the  changes  during  the  two  processes. 


"7     i 


The  human  ovum  comes  into  direct  apposition  with  only  a  small 
portion  of  the  uterine  wall,  and  the  changes  which  this  portion  of  the 
waU  undergoes  differ  somewhat  from  those  occurring  elsewhere 
Consequently  it  becomes  possible  to  divide  the  decidua  into  (i)  a 
^rtion  which  is  not  in  direct  contact  with  the  ovum,  the  decidm  vera 
(l-ig.  70,  V )  and  (2)  a  portion  which  is.    The  latter  portion  is  again 


130 


THE  DECIDUA  VERA 


capable  of  division.  The  ovum  becomes  completely  embedded  in 
the  mucosa,  but,  as  has  been  pointed  out,  the  chorionic  villi  reach 
their  full  development  only  over  that  portion  of  the  chorion  to  which 
the  belly-stalk  is  attached.  The  decidua  which  is  in  relation  to  this 
chorion  frondosum  undergoes  much  more  extensive  modifications 
than  that  in  relation  to  the  chorion  Ijeve,  and 
to  it  the  name  of  decidua  basalis  (decidua 
serotina)  (Fig.  76,  B)  is  applied,  while  the 
rest  of  the  decidua  waich  encloses  the  ovum 
is  termed  the  decidua  capsularis  {ilecidua 
reflexa)  (Q. 

The  changes  which  give  rise  to  the  decidua 
vera  may  first  be  described  and  those  occur- 
ring in  the  others  considered  in  succession. 

(a)  Decidua  vera. — On  opening  a  uterus 
during  the  fourth  or  fiHU  month  of  pregnancy, 
when  the  decidua  vera  is  at  the  height  of  its 
development,  the  surface  of  the  mucosa  pre- 
sents a  corrugated  appearance  and  is  traversed 


Fio.  78.— DuoSAiiHATic  Sections  of  ihe  Uterine  Mncos.*,  A   in  the  Non- 

PIEGNANT  UtEKUS,  AND  B,  AT  THE  BeOINNINO  OE  PkEOHANCY. 

c.  Stratum  compactum;  gl,  the  deepest  portions  of  the  glands;  m,  muscular  Uver- 
sp,  stratum  spongiosum.— (Ximirii/  and  EHgdmaimJ  ' 

by  irregular  and  rather  deep  grooves  (Fig.  77).    This  appearance  , 
ceases  at  the  internal  orifice,  the  mucous  membrane  of  the  cervix 
uteri  not  forming  a  decidua,  and  the  decidua  of  the  two  surfaces  of 
the  uterus  are  separated  by  a  distinct  furrow  known  as  the  marginal 
groove.     • 


I 


THl  DBCIDUA  CAPSCIAUS  j.j 

^n^f'^ZlXZZVr'  It''  '-"-  '^-'V  'Hick. 

from  the  appearance  which  it  nr^li.  'P^i"^um,  so  called 

and  conton^  i"  ons  oj  he  ZdV;"'""  °""« '° ''''=  •''■'^''='' 
and  (3)  ne«  thrmuscula  Jl,  oHh  'Z  "f  '"  '""""^  P'*""= 
contorted  but  no.  dilated "«  s  o"^^^^^^^^^^  r't"«  "^ 

mucosa  '^  "'"''"* ''^°'» 'he  connective  fssue  cells  of  the 

which  lie  in  the  stratum  complcmm  beclme  moT'  H  '  *'""'' 
Dressed  And  fi^.n,,  j-  !«"-'""•  oecome  more  and  more  com- 

E  much  fl  e'ZXr'"^  ""I  "^  ^'""«''«"'"  "^^  ^P-« 
at  fi^t  so  pr^'nirdrlirSt''  "^  '"^  '''°'  ''^^"^"''■ 
teniltf^ar^r-TJ^  decidua  capsulan-s  has  also  been 

-oftheute.::s^--rrtr^sr::^ 


'3* 


Tax   DECIDDA   BASAU8 


attached  itself  to  the  uterine  wall.  Since,  however,  the  attachment 
of  the  ovum  is  to  be  regarded  as  a  process  of  burrowing  into  the 
uterine  tissues  (see  p.  119),  the  necessity  for  an  upgrowth  of  a  fold  is 
limited  to  an  elevation  of  the  uterine  tissues  in  the  neighborhood  of 
the  ovum  to  keep  pace  with  its  increasing  size.  Since  it  is  part  of  the 
area  of  contact  with  the  ovum  it  possesses  no  epithelium  upon  the 
surface  turned  toward  the  ovum,  although  in  the  eariier  stages  its 
surface  is  covered  by  .in  epithelium  continuous  with  that  of  the 
decidua  vera,  and  between  it  and  the  chorion  there  is  a  portion  of 
the  blood  extravasation  in  which  the  villi  formed  from  the  chorion 
teve  float.  Glands  and  blood-vessels  also  occur  in  its  walls  in  the 
earlier  stages  of  development. 

As  the  ovum  continue  to  increase  in  size  the  capsularis  begins 
to  show  signs  of  degeneration,  these  appearing  first  over  the  pole 
of  the  ovum  opposite  the  point  of  fixation.  Here,  even  in  the  case 
of  the  ovum  described  by  Rossi  Doria,  the  cavity  of  which  measured 
6X5  mm.  in  diameter,  it  has  become  reduced  to  a  thin  membrane 
destitute  of  either  blood-vessels  or  glands,  and  the  degeneration 
gradually  extends  throughout  the  entire  capsule,  the  portion  of  the 
blood  space  which  it  encloses  also  disappearing.  At  about  the  fifth 
month  the  growth  of  the  ovum  has  brought  the  capsularis  in  contact 
throughout  its  whole  extent  with  the  vera,  and  it  then  appears  as  a 
whitish  transparent  membrane  with  no  trace  of  either  glands  or 
blood-vessels,  and  it  eventually  disappears  by  fusing  with  the  vera. 
(c)  Decidua  basalis.—The  structure  of  the  decidua  basalis,  also 
known  as  the  decidua  serotina,  is  practically  the  same  as  that  of  the 
vera  up  to  about  the  fifth  month.  It  differs  only  in  that,  being  part  of 
the  area  of  contact  of  the  ovum,  it  loses  its  epithelium  much  earlier 
and  is  also  the  seat  of  extensive  blood  extravasations,  due  to  the 
erosion  of  its  vessels  by  the  chorionic  trophoblast.  Its  glands, 
however,  undergo  the  same  changes  as  those  of  the  vera,  so  that  in 
it  also  a  compactum  and  a  spongiosum  may  be  recognized.  Beyond 
the  fifth  month,  however,  there  is  a  great  difference  between  it  and 
the  vera,  in  that,  being  concerned  with  the  nulrilion  of  the  embryo, 
it  does  not  partake  of  the  degeneration  noticeable  in  the  other  decidux. 


IHE  PIACENTA 


'33 


I 


the  maternal  tissues  and  is  the  decidua  basalis     »Zl  Vy,    . 
pans.     1  ne  fully  formed  placenta  is  a  more  or  less  discoidai  struc 

«en"LTr'the''tr'"""'''°'"'"'^'''''^^"=«'^'''«^<>^develop- 
em  DC  made,  the  foUowmg  structures  may  be  distineuishpH  •  On 

chorion '(c5'  .n  ihTch  the  de^  "'"^"'".'  '''''^"  """^  ^'"'^^  '^  "'^ 
may  be  ob^rved  *™'J ''>^/^8enerafve  changes  already  mentioned 

posed  nftr,'     .^""*^'''°8  ""«  "mes  a  much  broader  area  com- 

t^et-m  M  cTirtr'"""^ '"^  ^""'^  "■"  ^'^'''^  "■*  -'-"-f 

compac  urn  of    hi  h    7'      ''"°"'-    ^''*"  ^•'""^^  ">e  stratum 
next   ,r  *"*'"'   "*^*  «"=  st'-atum  spongiosum  fO'^ 

^     These  van-ous  structure,  have,  for  the  most  part,  been  already 


134 


Fro.  70.— SEcnoM  thsocob  a  Pucihta  of  Sivim  Momibs'  Devzlopuiit. 

Am,  Anmiao:  ekt,  chotkm;  D,  lijrer  of  deddu*  caotalnliig  the  uterine  riindtjirc, 
mnacdu  aMct  the  utera;  K«,  ostenul  bkiod-TCnel;  Vi,  itilk  a<  t  rHbt;  v^  iSm 
m  ■urtwiB  —yif ■!■<.) 


THE  PLACENTA 


^35 


tZt^J:""!  "  'Ti"'  ^'''  °°'^ '°  ^y  a  few  words  concerning  the 

changes  that  take  place  in  the  intervillous  space 

»,rJil^f *!'"")  TP*^'""  °f  t***  basa'  decidua  forms  what  is 
termed  the  basal  plaU  of  the  placenta,  closing  the  intervillous  space 
on  he  utenne  sjde  and  being  traversed  by  the  maternal  blood-vesTels 
that  open  mto  the  space.  The  formation  of  canalized  fibrin,  already 
mentioned  m  connection  with  the  decidua  vera  and  the  syncytium  of 

laverof  it  "^  """^  '\l^"  ^"^^  P""'""  °f  ">*  ^'''"^^'  *  definite 
layer  of  it,  known  as  Ntlabuch's  fibrin  stria,  being  a  characteristic 
constituent  of  the  basal  plate  and  patches  of  greater  or  less  exten 

cirrbr." ;  "''"^  °'  ^""^  p''^'"  ^^"-^^'^  ^^'^  --^  ■•» 

LLn  !!•''"""  '"  ""^  P'^'"=  "^^  *"^  P^^nce  has  been 
Uken  to  mdicate  an  attempt  on  the  part  of  the  maternal  tissues  to 

Ttl"  Tr:  ^^*'™  °^  '•'«  P^"'='«'=  ov">"-    From  the  surface 

mterviUous  space,  grouping  the  villi  into  cotyledons  and  giving 
attachment  to  some  of  the  fixation  villi  (Fig.  80).  Throughout  thf 
greater  extent  of  the  placenta  the  septa  do  not  reach  the  fu^ace  o 
H„  .1°"^'  "  *'  P'"P'"^^'  throughout  a  narrow  zone,  they 
Z^Z  "'Y."""? "  ^'h  the  chorion  and  unite  beneath  it  to  f;rm  a 

TtT^  ^^  '  """^  '""'•^  '^'  '*'«"^  ^'^-  Breath  this  lies 
toe  peripheral  portion  of  the  interviUous  space,  which,  owing  to  the 

Sr^  f  "''' '°  ""^  "^°"'  ^PP***-  '»  ^  ™I^rf-tty 
»^a«ted  from  the  r«st  of  the  space  and  forms  what  is  temed  the 
marginal  smus  (Fig.  80). 

fibnf  oTfiK"  ^^  "^^""^^  '^^  "^""^  '°  *'  f"™*"""  of  ^'""Ji^d 
ThrT.  ?  , '"  =°»°'*«°°  "ith  the  syncytium  of  the  viUi.  In 
mass^^,  m'S  °f  P^gnancy  there  may  be  produced  by  this  process 
masses  of  fibrinoid  of  considerable  size,  lying  in  the  interviUous  space; 

Sn  the  ,  T"*,  "  """■^'  "'  ^^''^'^'^  P'''^*'-  though  thl 
tie  H  »       ^r  r'"  """  "'  "P"'^'°"-    ^"^  ^S<"'i'  produced  by 


SEPARXnON  OF   THE  DECIDU* 


137 


The  Separation  of  the  Decidiue  at  Blrth.-At  parturition, 
after  the  rupture  of  the  amnion  and  the  expulsion  of  the  fetus,  there 
still  remain  in  the  uterine  cavity  the  deciduae  and  the  amnion,  which 
IS  m  contact  but  not  fused  with  the  decidua^.  A  continuance  of  the 
utenne  contractions,  producing  what  are  termed  the  "after-pains  " 
results  in  the  separation  of  the  placenta  from  the  uterine  walls  the 
separation  taking  place  in  the  deep  layers  of  the  spongiosum,  so 
that  the  portion  of  the  mucosum  which  contains  the  undegenerated 
glands  remains  behind.  As  soon  as  the  placenta  has  separated 
the  separation  of  the  decidua  vera  takes  place  gradually  though 
rapidly,  the  line  of  separation  again  being  in  the  deeper  layers  of  the 
stratum  spongiosum,  and  the  whole  of  the  decidus,  together  with 
the  amnion,  is  expelled  from  the  uterus,  formi..^'  what  is  known  as 
the  "after-birth." 

Hemorrhage  from  the  uterine  vesseb  during  and  after  the  separa- 
tion of  the  deciduffi  is  prevented  by  the  contractions  of  the  uterine 
walls,  assisted,  according  to  some  authors,  by  a  preliminary  blocking 
of  the  mouths  of  the  uterine  vessels  by  certain  large  polynuclear 
decidual  cells  found  during  the  later  months  of  pregnancy  in  the  outer 
layers  of  the  decidua  basalis.  The  regeneration  of  thcuterine  mucosa 
after  parturition  has  its  starting-point  from  the  epithelium  of  the 
undegenerated  glands  which  persist,  this  epithelium  rapidly  evolving 
a  complete  mucosa  over  the  entire  surface  of  the  uterus. 

th.^L^^f^"^^!'^  arrangement  of  the  human  placenta  is,  of  course, 
^e  cu^mmauon  of  a  long  series  of  specializations,  the  path  iong  whfch 

^ ^Z,  •'^'  '°''''  r "T^-  'T*''  Monotremes resemble  the reptlj 
rvum^^r''"?"'  ^^^^  group  of  forms  there  is  no  relation  of  h^ 
?r,T,  ^  ^'""^  tissues  such  as  occurs  in  the  formaUon  of  a  placenta 
jL^nH  ""^-^ab  viviparity  is  the  rule  and  this  condition  does 

On?of  th'^^Zr.°S~'"r'^°"  '•"i^^*"  '^"=  ^'"^  '"«'  maternal  Lues 
une  of  the  simplest  of  such  connections  is  that  seen  in  the  oie  where  th.. 

merinl^  °'.h'^^.-°™"'k*'  '"'°  corresponding  depSons  b  £ 
bLS  ^r^??  '•'^,'^"^^J'°''*^"'  "-dergoing  no  destruction,  and  at 
leaWne^U^ct"T^  ^"^^T,  *"""  f^e  depressions  of  the  mucosa 
^^ItTul  ^  ^P*  °/  placentation  is  an  embryotrophic  one,  and 
n°^  i,  •  1  no  separation  of  decidua  from  the  uterine  wall  after  preg- 
nancy It  IS  also  of  the  i„<Ucidual,  tyfe.    In  the  sheep  the  placentaUon  !s 


138 


tITERATUUE 


also  embryotrophic  and  indedduate,  but  destruction  of  the  maternal 
mucosa  does  take  place,  the  villi  penetrating  deeply  into  it  and  coming  into 
relation  with  the  connective  tissue  surroun^g  the  maternal  blood-vessels. 
Another  step  in  advance  is  shown  by  the  dog,  in  which  even  the  con- 
nective tissue  around  the  maternal  vessels  in  the  placental  area  undergoes 
almost  complete  destruction  so  that  the  chorionic  villi  are  separated 
from  the  maternal  blood  practically  only  by  the  endothelial  lining  of  the 
maternal  vessels.  In  this  case  the  mucosa  undergoes  so  much  alteration 
that  the  undestroyed  portions  if  it  are  sloughed  off  after  birth  as  a  deddua, 
so  that  the  placentation,  like  that  in  man,  is  of  the  deciduale  type.  It 
still  represents,  however,  an  embiyotrophic  type,  although  closely  appioxi- 
mating  to  the  luemotrophic  one  found  in  man,  in  which,  as  described  above, 
the  destruction  of  the  maternal  tissues  proceeds  so  far  as  t-^  open  into  the 
maternal  blood-vessels,  so  that  the  fetal  villi  are  in  direct  contact  with  the 
maternal  blood. 

If  these  various  stages  may  be  taken  to  represent  steps  by  which 
the  conditions  obtaining  in  the  human  placenta  have  been  evolved,  the 
entire  pre .  "ss  may  be  regarded  as  the  residt  of  a  progressive  activity  of  a 
parasitf.e  .<' um.  In  the  simplest  stage  the  pabulum  supplied  by  the 
uterus  was  sufficient  for  the  nutrition  of  the  parasite,  but  gradually  the 
ovum,  by  means  of  its  plasmodi-trophoblast,  began  to  attack  the  tissues 
of  its  host,  thus  obtaining  increased  nutrition,  until  finally,  breaking 
through  into  the  maternal  blood-vessels,  it  achieved  for  itself  still  more 
favorable  nutrition,  by  coming  into  direct  contact  with  the  maternal 
blood. 


LITERATURE. 

In  addition  to  the  papen  by  Beneke  and  Sttahl,  Biyce  and  Teacher,  Fraaa.'   'ung, 
and  Herzog,  cited  in  Chapter  III,  the  following  may  be  mentioned: 

E.  CovA:  "  Ueber  ein  mensdilichei  Ei  der  zweiten  Woche,"  Arch./tr  Cynaek.,  ixxxni, 

1907. 
L.  FlASSI:  "Ueber  ein  junges  menschliches  Ei  in  3itu,"  Arck.  far  mikr.  Anct.,  Lxx, 

1907. 
O.  Giossu:  "Vergleichende  Anatomie  und  Entwicklnngigeschichte  der  Eihlute 

und  der  Placenta  mit  bcsondeier  Beiflcksichtigung  des  Menschen,"  Wien,  1909. 
H.  Happi:  "Beobachtungen  an  Eihiuten  jimger  menschlichei  Eier,"  Anal.  HefCt, 

xxxn,  1906. 
W.  Hd:  "Die  Umacbliessung  det  menschlichen  Frucht  wihicnd  der  {rtlhesten  Zeit. 

des  Schwangenchafts,"  Animfttr  Anat.  und  PkytM.,  Anat.  AM.,  1897. 
M.  Honcxixit:  "  Die  menschliche  Placenta,"  Wiesbaden,  1890. 

F.  Knaat,:  "Zur  Entwickelungigeschichte  der  Placenta,"  Anal.  Anteiftr,  iv,  1889. 

J.  KOLUIANH:  "  Die  menschlichen  Eier  voo  6  nun.  Gr6sse,"  Arckh  ftir  Atul.  tmd 
Pkytud.,  Anat.  AUk.,  1879. 

G.  Leopold:  "  Ueber  ein  lehr  junga  menichUchct  Ei  in  litu,"  Arb.  atu  dtr  tunifl 

FtaumUmlk  in  DraJen,  nr,  1906. 


F.  >^CH*N»:  ••B«*.chn«,g«  «  ,^^  „„«.hlkha,  Ei™,"  ^««,.i„/<,.  ,„, 
^"«:  "U«b«'  die  Einbrttung  d«  mraschlich™  Ei«  und  dw  frth«te  hi.l.„ 

"r^^-^tLx"^""^*^-'  ''"""''""■•■•'  ^'•■-*"/'/-  c.*.'.**;/.  una 

'•  "r/r^i^s::  ^n^""*- '''™-" """ "-"-  ««-■"  -"»-««■ 

__  Neura  ttber  den  Bau  dw  Placenta,"  Md,  vi,  1897.  '™^" '»93. 

"Placentaranatomie,"  jW.  vni  1899 

J.  C.  Webstm:  "Human  Placentation,"  Chicago  1901 


PART  II. 
ORGANOGENY. 


CHAPTER  VI. 
Tffli  BEVEWMBHi  OF  THE  BTEODllMTMr  SYSTEM 

The  ectodena  covering  the  general  surface  of  the  body  is  in  .h, 
earhest  stages  of  developn^ent.  a  single  layer  of  cells  bmat'he  et 

eJtu        """^^  "  ''  '"""^""^  °'  '-»  "'y*".  an  outer  one  fte 
'P^krum.  consisting  of  slightly  flattened  eel! s   and  a   ower'one 

the^„! ;         K       ^  '^^  '*^°'"^  """"^  '•>«  differences  between 
he  two  layers  become  more  pronounced,  the  epitrichial  cells  a  Z 

(^ig.  81,  B).  These  cells  persist  until  about  the  sixth  month  of 
d  velopment,  but  after  that  they  are  cast  off,  and,  becomirmLd 
wuh  tie  secretion  of  sebaceous  glands  which  have  appear  d  by  Tw' 
time,  form  a  consutuent  of  the  vernix  caseosa  ^ 

ayer  which  result  m  its  becommg  several  layers  thick  (Fig  8i  B^ 
the  innermost  layer  being  composed  of  ceL  rich  in  prlplsm' 

,!h  !k        ^f,'*«^«'°P'»«nt  proceeds  the  number  of  layers  increases 

o  the  ItLT     "'  """'  ""'^^°^«  ^  ''""^y  degeneration,;":  ris" 
to  the  stratum  corneum,  while  the  deeper  ones  become  th;Ttra.um 

HI 


148 


DxvxLonoMT  or  tkc  ixin 


Malpighii.  At  about  the  fourth  month  ridges  develop  on  the  under 
surface  of  the  epidcfrmig,  projecting  downward  into  the  dermis, 
and  later  secondary  ridges  appear  in  the  intervals  between  the 
primary  ones,  while  on  the  palms  and  soles  ridges  appear  upon  the 
outer  surface  of  the  epidermis,  corresponding  in  position  to  the 
primary  ridges  of  the  under  surface. 

The  mesenchyme  which  gives  rise  to  the  dermis  grows  in  from 
all  sides  between  the  epid.  rmis  and  the  outer  layer  of  the  myotomes, 


Fio.  Si.— A,  Sectios  op  Skin  from  the  Doksoii  or  Finoer  op  an  Ehbiyo  or  4  c  ch  ■ 
B,  raoii  THE  Plantar  Surface  of  the  Foot  of  an  Ehbrvo  of  io.j  ci     '' 
et,  Epitrichium;  ep,  epidermis. 

which  are  at  first  in  contact,  and  forms  a  continuous  layer  under- 
lying the  epidermis  and  showing  no  indications  of  a  segmental 
arrangement.  It  becomes  converted  principally  into  fibrous  con- 
nective tissue,  the  outer  layers  of  which  are  relatively  compact, 
while  the  deeper  ones  are  looser,  forming  the  subcutaneous  areolar 
tissue.  Some  of  the  mesenchymal  cells,  however,  become  converted 
into  non-striated  muscle-fibers,  which  for  the  most  part  are  few  in 
number  and  associated  with  the  hair  follicles,  though  in  certain 
regions,  such  as  the  skin  of  the  scrotum,  they  are  very  numerous  and 


OKVnoPIIENT  or  THI  SKIN 

fonii  a  distinct  layer  known  as  the  dartos 
Some  cells  also  arrange  themselves  in  groups 
and  undergo  a  fatly  degeneration,  well-defined 
masses  of  adipose  tissue  embedded  in  the 
lower  layers  of  the  dermis  being  thus  formed 
at  about  the  sixth  month. 


//>/ 


Tn 


t-' 


AlAough  Uie  dermal  me«nchyme  i,  unseg- 
branches  to  it  are  segmental,  and  it  mieht  be     k      /i 

adult  Tin'  *''A^""~'»  n«rves  even  in  the 

^f^l  ^?^  °^  ^^  cutaneous  nerve-supply  i„ 

the  adult  realizes  to  a  veiy  considerable  went 

^  expecution,  the  areas  supplied  by  the  vSous 

nmes  forming  more  or  les,*^  distinct  zon^.  and 
bmg  therefore  segmental  (Fig.  82).  But  a  con 
t^  "'""^ng'ing  of  adjfcen/areas  {.«X 
^i^"  K  J'  *'"''  ^^  distribution  of  the 
cutwieous  branches  of  the  fourth  thoradc  nenTe 
ys  determined  experimentally  in  Se  monkey 
(Macacos),  ,s  distincUy  zonal  or  segmen™lhe 
mpple  lymg  practicaUy  in  the  mid^™e  of  fte 
«.ne,  the  upper  half  of  its  area  is  also  suSphvfd  or 

lower  haU  by  fibers  of  the  fifth  (Fig.  83),  »  tSt 
any  area  of  skin  in  the  zone  is  inner^ted  by  fib«i 

M^^L      ''■  f""''".?"'.  the  distribution  of 
each  nerve  crosses  the  mid-ventral  Une  of  the  body 

S  "noTf ."•''^  "''°''™  cross,Sov:rU^^' 
»r..  K  .  "' "'"'''  "  ''*"«  "  confusion  of  adjacent 
^  but  an  area  may  shift  its  position  reSS 

nerve  s^TAlf'T""*  '"'PP'''^  ^^  ^  »ae 
!!^'  ^1      '  ^^f^^  over  a  certain  muscle  is  not 

nau  of  the  abdomen,  the  skin  at  any  point  will 

UMCraiJ.  OOSDiSTMBOnONOIIB* 


li 


144 


DEVZIOPMIEMT  OF  THX  NAILS 


The  DeTelopment  of  the  Halle.— The  earlieat  indications  of 
the  development  of  the  naiU  have  been  described  by  Zander  in 
embryos  of  about  nine  weeks  as  slight  thickenings  of  the  epidermis 


\ 


iiuMmiiimiiiiim 


xmimimmmmm 
mmmmmm^ 


\m\\\\m\\\\mm\\mN 

Fio.  83.— DuoxAH  SHOwiNa  TBI  OvxuAF  OF  TBI  III,  IV,  ud  V  IxmcogTU 
NiKvil  or  A  MoMnty. — (^ktrrmffon.) 


FtO.   84. — LONCirCDDIAL  SXCnOH  THSOnOH  THE  TZUONAL  JOIHT  OT  THE  InSEX- 
FlNGEK  OF  AN  ElfBRYO  OF  4.5  CK. 

t,  Epidemus;  tp,  epitrichium;  ff/',  nail  fold;  Pk,  tenninal  phalanx;  zp,  sole  plate. 

of  the  tips  of  the  digits,  these  thickenings  being  separated  from  the 
neighboring  tissue  by  a  faint  groove.  Later  the  nail  areas  migrate 
to  the  dorsal  surfaces  of  the  terminal  phalanges  (Fig.  84)  and  the 


Jif- 


ep- 


MVBlOMttNT  or  THE  NAIIS  „, 

temed  soU-plaUs  (sp),  structures  quite  mi" 
mentan.  m  man,  but  largely  developed  in  the 

The  actual  nail  substance  does  not  form 
however,  until  the  embryo  has  reached  a  len^.' 
of  about  17  cm.    By  this  time  the  epidermis  has 
become  several  layers  thick  and  its  outeHayen 
over  the  nail  areas  as  well  as  elsewhere   £ 
^come  t^nsformed  into  the  stratum  co^eum 
Fig.  &s,  sc),  and  it  is  in  the  deep  layers  of  this 
(the  stratum  lucidum)  that  keradn  ^anules  de 
velop  .„  cells  which  degenerate  to^ve  rfse  to 
the  nail  substance  („).     At  its  first  formau^i 
accordingly  the  nail  is  covered  by  the  outer  la^^' 
of  the  stratum  corneum  as  well  as  by  the  eoi 

Sn^lftK^^"  "^^"-^^  forming  U'a^s 
been  termed  the  eponychium  (Fig.  Sc  et)  Th,. 
epitrichium  soon  disappear,  howevef  ,e  Jn_ 
only  the  outer  layers  of  the  stratum  comeum  a! 

err'  "^  *'l°  '^-  ''-PP-  wiZh 
exception  of  a  narrow  band  surrounding  the  base 
of  the  nail  which  persists  as  .he^«,;„l 

pro^mll  ™.'"°"  f  '^'  """  ^*«^«  '»  'he  more 
proxima  portion  of  the  nail  area  and  its  further 
growth  IS  by  the  aHHitmn   ^1  .       7""" 

celU  M  !f  •  *T"°"  °f  °ew   keratinized 

.h         M  u  ?'"''"""'  ^"^'-'^  ^"d  'ower  surface 

cti7teti^=r"^'''^^-''-^'> 


Fio.  85.— LoKoi- 

TUDINAL  SiCnOK 
™»OUOH  THl  NAtt 
Akia  in  an  EUBIYO 
0»  17  CM. 

*P,  Bponychium; 
n,  nail  luUtance;  ni, 
nail  bed;  «,  atratum 
corneum;  sp,  sole 
plate.— (O/tanwra.) 


ufi 


OIVCLOrMKNT  or  THE  KAIU 


confirmed  by  later  '^^aerven,  but  the  migntion  of  the  areas  to  the  dorsal 
iurf  ace  neccssilatcil  by  such  a  location  of  the  prinutry  differentiation  affords 
an  explanation  of  the  otherwise  anomalous  cutaneous  nerve-supply  of  the 
nail-areas  in  the  adult,  this  being  from  the  palmar  (plantar)  nerves. 

The  Development  of  the  Hairi.— The  hairs  begin  to  develop 
at  about  the  third  moitth  and  continue  to  be  formed  during  the 
remaining  portions  of  fetal  life.  They  arise  as  solid  cylindrical 
downgrowths,  projecting  obliquely  into  the  subjacent  dermis  from 


Fio.  86.— The  Diyklopiiznt  or  a  Hah. 
c,  Cylindrical  cells  of  suatum  mucoaum;  V>  *^11  °^  ^^'^  tolUcle;  m,  mesoderm; 
tmt,  stratum  mucosum  of  epidermis;  p,  hair  papilla;  r,  root  of  hair;  x,  sebaceous  gland. 
— (KcUmamt.) 

the  lower  surface  of  the  epidermis.  As  these  downgrowths  continue 
to  elongate,  they  assume  a  somewhat  club-shaped  form  (Fig.  S6,A), 
and  later  the  extremity  of  each  club  moulds  itself  over  the  summit  of 
a.  small  papilla  which  develops  from  the  dermis  (Fig.  86,  B).  Even 
before  the  dermal  papilla  has  made  its  appearance,  however,  a 


MvnoMowT  or  m  baiu  ^^ 

one  a«„«e  a  cuwSol  .„H         '•'  ""''l  ""=  """"••  •""'•'•^■"' 
follicle     Tt,.  f  Z.U  '"••  constitute  the  lining  oi  the  hair 

oapllla.  '""'"P""'^°»  *"  ^'  epidermal  cell,  which  envelop  the 

wS.  U  a,  ^.  tlir*^-?'  "l'*^™"  '°  '°™  "^  sebaceous  gland, 
low     T^eStr^  .„'"''  f^''''^'  'h-ih  later  it  become 
ipheral  and  fZ  °l  ""*  °"'«'°*"'  ''P"^"''  from  the  per- 

foUide  walls  and  the  hairTn  I  T  'P""  *'"""'"  ">« 

cell,  latergiving  ri  bv  LiSn  .'  ""^  '"^''"'  ""=  P*"""""' 

During  fetal  life  th^f.^  "  """  generations  of  central  cells. 

of  the  bS  iloml  ™'  "f^"'^' '""^  P"""''  ""'  "P°"  'he  surface 
constL^'th^whTtror"^  ""V''  "^'-°*'  '^P""'^'"'"  ""«  -d 
covers^  urface  of  he''"°"l!"''''"'''  ""^  "'"^  '""-"•  "hich 
^iton...    conn«^,It        K  "u  *"'""'  '''"'*•     The  muscles,  arreCores 

anHtty  and  c„T«-l  !•    ^u"'"'''  '""P"*^ ''  »>«'"8  exceedingly  fine 

noticeable  r:c  oun.  „?;;  SeT  ""^  '"';  :'"'"  "  "  ^^'^ 
The  coarser  haira  whirl  T  ?°""^  '^'^  '^'^^  "^  coloration. 
probabHriserm!  ^  n"^  *"  "  '"  "^'"■"  '^«'°"«  »'  ">«  body 
place  LuXmreT"d""'''''°™''''°"°"°'''^'-'''t^ 
birth.  But  even  tht^f  T"*^'  °'  ''""  "'^  '"«'  P^^^'^'y  a^er 
'or  any  g^at  ZS      "  k™*^  '""  ""  ""'  individually  persist 

■"  a  hair  is  pr«^Th  "^  "^.'"''  '"  '''='"'  P"""^"'     "^"^  ''"^ding 

coven>i  L  ^    ^  %r'""°"  "'  "•^  proliferation  of  the  cells 
enng  the  dermal  papdla  and  by  a  shrinkage  of  the  papilla 


(t 


14ft 


OEVEtOPUENT  OF  THE  SCDOSIPAXOUS   GLANDS 


whereby  it  becomes  detached  from  the  hair,  and  the  replacing  hair 
arises  from  a  papilla  which  is  probably  budded  off  from  the  older 
one  before  its  degeneration  and  carries  with  it  a  cap  of  epidermal 
cells. 

It  is  uncertain  whether  the  cases  of  excessive  development  of  hair 
over  the  face  and  upper  part  of  the  body  which  occasionally  occur  are 
due  to  an  excessive  development  of  the  later  hair  follicles  (hypertrichosis) 
or  to  a  persistence  and  continued  growth  of  the  lanugo. 

The  DeTelopment  of  th«  Sudoriparous  Glands.— The  sudor- 
iparous glands  arise  during  the  fifth  month  as  solid  cylindrical  out- 
growths from  the  primary  ridges 


M^y^-' 


— * 


of  the  epidermis  (Fig.  87),  and 
at  first  project  vertically  down- 
ward into  the  subjacent  dermis. 
Later,  however,  the  lower  end  of 
each  downgrowth  is  thrown  into 
coils,  and  at  the  same  time  a 
lumen  appears  in  the  center. 
Since,  however,  the  cylinders  are 
formed  from  the  deeper  layers 
of  the  epidermis,  their  lumina  do 
not  at  first  open  upon  the  sur- 
face, but  gradually  approach  it 
as  the  cells  of  the  deeper  layers 
of  the  epidermis  replace  those  which  are  continually  being  cast  off 
from  the  surface  of  the  stratum  cometun.  The  final  opening  to 
the  surface  occurs  during  the  seventh  month  of  development. 

The  Development  of  the  Mammary  Glands.— In  the  majority 
of  the  lower  mammals  a  number  of  mammary  glands  occur,  ar- 
ranged in  two  longitudinal  rows,  and  it  has  been  observed  that  in  the 
pig  the  first  indication  of  their  development  is  seen  in  a  thickening 
of  the  epidermis  along  a  line  situated  at  the  junction  of  the  abdomi- 
nal waUs  with  the  membrana  reuniens  (Schulze).  This  thickening 
subsequently  becomes  a  pronounced  ridge,  the  miXk  ridge,  from 
which,  at  certain  points,  the  mammary  glands  devdop,  the  ridge 


Fio.  87.— LowiB  Sduace  or  a  De- 
tached POKTION  OV  EPIDXUUS  FROM 
THE  DOBStni  OF  THE  HAND. 

h.  Hair  foUide;  s,  tudoripannis  gland. — 
(BAucUo.) 


2  below  the  axn,a  to  tiXuti  ^ej^^l^^ssta;?"'^  ^™'" 
ently  equivalent  to  the  milk  line  of  the  p"^  a „H  f  ^  "*'  "^P*'" 
IS  mm.  the  upper  end  of  the  line  i1h  ^  *""  '^°'  °^  "*  °' 

while  more  posteriorly  <he  h  c  W  ha^T'  "  ''™"°""^"^  "<1««' 

The  further  history  of  the  wi^  ^"^PP*^'"^- 
traced  in  human  embls  aid. h*'    "'  ""*'  '"''"'''•  "^^^  y^t 
the  glands  which  hasTl  ob      '  "       '''"'  °'  '"'  development  of 
served  is  one  in  which  they  are 
represented  by  a  circular  thick- 
ening of  the  epidermis  which 
■       projects    downward    into    the 
dermis   (Fig.  89,  A).       Later 
the  thickening  becomes  lobci! 
(Fig.  89,  B),  and  its  superficial 
and  central  cells  become  comi- 
fied  and  are  cast  off,  so  that  the 
gland  area  appears  as  a  depres- 
sion of  the  surface  of  the  skin 
During    the    fifth    and   sixth 
months  the  lobes  elongate  into 
solid  cylindrical  columns  of  cells  — «vo.-(jr««A„) 


■mr 


Ekbevo.— (jrotfnu.) 


ISO 


rEVZLOFHXNT  OF  THE  MAmUKY  GLANDS 


the  extremities  of  the  lacteal  ducts.  In  the  areola,  as  the  area  sur- 
rounding the  nipple  is  termed,  other  glands  known  as  Montgomery's 
glands,  also  appear,  their  development  resembling  that  of  "the  mam- 
mary gland  so  closely  as  to  render  it  probable  that  they  are  really 
rudimentary  mammary  glands. 


.X-T^YTTVi'.C'i ,-  ■'.  ]    A 


Fio.  89— Sections  tbsocch  the  Epidziiiul  THiCKEinNos  which  Rifuzsent  the 
Mahhary  Gland  in  Ehbiyos  (.4)  of  6  cm.  and  (B)  of  to.3  cm. 

The  further  development  of  the  glands,  consisting  of  an  increase 
in  the  length  of  the  ducts  and  the  development  from  them  of  addi- 
tional acim',  continues  slowly  up  to  the  time  of  puberty  in  both  sexes, 
but  at  that  period  further  growth  ceases  in  the  male,  while  in  females 
it  continues  for  a  time  and  the  subjacent  dermal  tissues,  especially 
the  adipose  tissue,  undeigo  a  rapid  development. 


LTTEaATDKE 

fa  associated  with  the  formarim.  „f  "^  °°"^^  development  or 

Thi.  fa  by  no  means  :„i^?^q:^„°'j„^^^^^^  «'ands  (polUsii^). 

percent  of  over  100,000  soS  of  th^r^i^'  "  ''"'  ''f "  ''•>«"ed  in  .9 
and  occurs  in  47  Per^monnHiWI^.^-^"  ""^  "'''"""e  ""mined' 
The  extent  to  Sthe  anomi?v1ri"^f  '"5'""."  ™P<>°*  "f  German; 
of  well-developed  acc«X  2nrf,  .  ?r.''  ^f"''  '"""  **  occurrenc^ 
nppl«(*y^tti»arS7attt-»m^^^^^  nidimentary  acce,«,^ 

of  a  normal  gland  ak'd  b^g  S^dr,?  ^,ru"""^ '"  "^^  "~'"  «"'» 
of  one  or  more  of  Montgome^?,  gK' '"  '"'''  =""  '"  »"  hypertrophy 


FlO.    9a— SzCnOH    THUOTOH    THE    MaiOUIV    f-r.^ 

fem^'^°:«L*'AH^?Siattcc"^^^^^^^^^^  '"-'--^  ""^-y  'n 
unknown  in  which  the  glands  have  lien  w.l^lJ^?^'?''^'  "^  "«  "ot 
males  (gynacomaslia).    Furthermnr.  f  7*" '^<^«  "Ped  and  functional  in 

normally  occurs  imm^dia.dy'aftrb  ;tj  ?«"!';" of  Ch'*^  •"  '^.'.«""'''' 
tew  drops  of  a  milky  fluid  the  ■m  r,n^  "^}^Ms  of  both  sexes  yieldinR  a 
the  gland,  are  subj^ted  to  prL^;^^"'"' '"'"'-"""'  ("""milS).  wh^en 

LITERATURE. 
J.  T.  ^wx.  "  The  Epitrichia,  W„  <.  ^  H„™„  ^^^,  ,^  ^^^  ^ 

O.  B™:™„:  ..„.,„  .„,^„^.  ^^^_^^^  ^__^  ^^^  _  ^^   ^^^^ 
"■  ^e™i  nLL'^^rx^  'Z'ZltJ'-,^'  '^'"™"  •»  *'  «.  of 


it 


iS» 


LITERATUKE 


T.  Okakd**:  "Ueber  die  EntwicUung  det  Nageb  bdm  Mrauchoi,"  Arckiv  fir 

Dmulal.  undSytUlel.,  xxv,  1900. 
H.  SCHHIDT^  "Ueber  nomule  Hyperthdie  menschUcht.-  Embiyontn  und  ttber  die 

ente  AnUge  der  menschlichen  MilchdiUsen  Oberh»upt,"  Uortkal.ATttUm  xvn 

1897. 
C.  S.  Sbmmnotom:  "Eiperimenla  in  Eimmination  of  the  Peripheral  Dbtribution  of 

the  Fibies  of  the  Posterior  Roots  of  some  Spinal  Nerves,"  Pkilo!.  Trans.  Raytl 

Soe.,  cixxxiv.  1803,  and  cxc,  1898. 
P.  Siohr:  "Entwickelimgsgeschichte  des  menschlichen  Wollhaarcs,"  Anal.  Heflt 

xxin,  1903.  ' 

H.  Stiabi.:  "Die  eiste  EntwicUung  der  Mammarorgane  beim  Menschcn,"  Vmhtmil. 

Anal.  Caellsch.,  xn,  1898. 


CHAPTER  VII 

termed  connective  ^"rjiinJVl:  ™°'"*"*  '°*°  "'^^  «'« 
of  a  non-cellu  J  Iwi^^wh-  f "'''  ^'«"''=''"'^  by  consisting 
embedded.    tTJ^W ""Te  't    """"  "  ''"  "^""""^  «"«  "« 
fonnation  of  auZ   ^s"  ^1.:,^^  ^^  '"^  ~  '"'°  "'^ 
fonning  the  central  ne^ous  system^H  7.^"  '^"'"""'  °^  "-"^ 
lK.l.b^ether  and  supporr^'^'nrorXh  the'r  '^'""' 
composed;  in  addition,  they  take  the  iol^JT^        ^""^  ^"= 
(seious  membranes,  fascia;)    corfl  LT      ,^'"  "'""branes 
-sses  (cartilage),  or t^L^fm  L^o^Cr ^ '  °'  =*'"''^ 
spongy  texture  (areolar  tissued     TW-  T      ."  "^  *  somewhat 
somewhat  varied  in  X^actert^J      «'"?«»■«««  substance  is 
non-branching,  noLekstfcfiL^   *  '"T"^  ^'"^"■"^^  °*  -hite, 
elastic  fibers'!^;,  itt^XXr^LJc  ST'  ^kT^' 
a  reticulum,  or  of  a  soft  Kelatinou,«„K!,  •         *''"'''  '°"n 

quantities  of  mucin,TsnSrsuel-^"'^^^ 
jelly  of  the  umbilkal  cor^    Z  „  °^"""''' ""=  ^^°'"'''" 

pact  and  homo^neous   or  ^her  """  '''  '"^'"'^  '^  ™'»- 

Passing  over  into^r^"fib«  JeTndT  ^  '""  ''"°"^' 
o^anie  matn.  is  ia^eH^prer^rtrr^tSe""  '^"'^  '^^ 

actual  -^^oZlT:/lf':^i:^i;;J]^^^  "y  the 

othe^c-l^.thatrtisma.„acturedbyL:eS„;t^::rdiS 

'53  ^ 


IS4 


vtyKLonam  or  connective  tisscz 


represent  the  cells  themselves.  Fibrils  and  material  out  of  which 
fibrils  could  be  formed  have  undoubtedly  been  observed  in  connec- 
tive-tissue cells,  but  whether  or  not  these  are  later  passed  to  the 
exterior  of  the  cell  to  form  a  onnective-tissue  fiber  is  not  yet  certain, 
and  on  this  hangs  mainly  the  difference  between  the  theories. 
Recently  it  has  been  held  (Mall)  that  the  mesenchyme  of  the  embryo 
is  really  a  syncytium  in  and  from  the  protoplasm  of  which  the  matrix 


Fio,  91 


-PO«TION  or  THE  CeKTEH  Of  OsSmCATION  Ot  THE  PaEIETAL  BoMZ  OF  A 

Human  Ehbeyo. 


forms;  if  this  be  correct,  the  distinction  which  the  older  views  make 
between  the  intercellular  and  intracellular  origin  of  the  matrix 
becomes  of  little  importance. 

Bone  differs  from  the  other  varieties  of  connective  tissue  in  that 
it  is  never  a  primary  formation,  but  is  always  developed  either  in 
fibrous  tissue  or  cartilage;  and  according  as  it  is  associated  with  the 
one  or  the  other,  it  is  spoken  of  as  membrane  bone  or  cartilage  bone. 
In  the  development  of  membrane  bone  some  of  the  connective-tissue 
cells,  which  in  consequence  become  known  as  osteoblasts,  deposit 
lime  salts  in  the  matrix  in  the  form  of  bony  spicules  which  increase 
in  size  and  soon  unite  to  form  a  network  (Fig.  91).  The  trabeculse 
of  the  network  continue  to  thicken,  while,  at  the  same  time,  the  forma- 
tion of  spicules  extends  further  out  into  the  connective-tissue  mem- 
brane, radiating  In  all  djiectiotts  fftm  the  region  in  which  it  first 


DEVELOPIIEMT  OF  BOOT:  j.j 

developed.  Later  the  connective  tissue  which  lies  upon  either  sur- 
face o  the  reticular  plate  of  bone  thus  produced  condenses  to  form 
as  out  membrane,  the  periosteum,  between  which  and  the  osseous 
plate  osteoblasts  arrange  themselves  in  a  more  or  less  definite  layer 
and  deposit  upon  the  surface  of  the  plate  a  lamella  of  compact  bone 
A  membrane  bone,  such  as  one  of  the  flat  bones  of  the  skull,  thus 
comes  to  be  composed  of  two  plates 
of  compact  bone,  the  inner  and 
outer  tables,  enclosing  and  united 
to  a  middle  plate  of  spongy  bone 
which  constitutes  the  diploe. 

With  bones  formed  from  carti- 
lage the  process  is  somewhat  dif- 
ferent.     In    the    center    of   the 
cartilage  the  intercellular  matrix 
becomes  increased  so  that  the  cells 
appear  to  be  more  scattered  and 
a  calcareous  deposit  forms  in  it. 
All  around  this  region  of  calcifica- 
tion the  cells  arrange  themselves 
in  rows  (Fig.  92)  and  the  pibcess 
of  calcification  extends  into  the 
trabeculae  of  matrix  which  separate 
these  rows.    While  these  processes 
have  been  taking  place  the  mesen- 
chyme surrounding  the  cartilage 
has    become    converted    into    a 

periosteum  {po),  similar  to  that  of  membrane  bone,  and  its  osteo- 
blasts deposit  a  layer  of  bone  ^  upon  the  surface  of  the  cartilage 
I  he  cartilage  cells  now  disappear  from  the  intervals  between  Uie 
trabecule  of  calcified  matrix,  which  form  a  fine  network  into  which 
masses  of  mesenchyme  (Fig.  93,  pi),  containing  blood-vessels  and 
^teobksts  here  and  there  penetrate  from  the  periosteum,  after 
hanng  broken  through  the  layer  of  periosteal  bone.  These  masses 
absorb  a  portion  of  the  fine  calcified  network  and  so  transform  it 


FlO  93 —LOKOmiBINAL  SECTION  or 
J-HALANX  OF  A  FraGM  or  AN  EUBIYO 

ot  3  i/a  Months. 

c.  Cartilage  trabecute;  p,  periosteal 
bone;  /w,  periosteum;  x,  ossification 
center.— (5iyii<oiKiiCTi:,.) 


156 


DEVZlOPlIEirr  OF  BONE 


Into  «  coane  network,  the  meshes  of  which  they  occupy  to  form 
the  bone  marrau,  («,).  and  the  osteoblasts  which  they  conufn  Zr^Z 

layers  of  bone  upon  their  surfaces.    In  the  meantime  the  calcifca 
t.on  of  the  carriage  matrix  has  been  extending,  and  as  fasTaT^e 

network  of  calcified  trabecuUe  is 
formed  it  is  invaded  by  the  mesen- 
chyme, until  finally  the  cartilage 
becomes  entirely  converted  into  a 
mass  of  spongy  bone  enclosed 
within  a  layer  of  more  compact 
periosteal  bone. 

As  a  rule,  each  cartilage  bone 
is  developed  from  a  single  center 
of  ossification,  and  when  it  is  found 
that  a  bone  of  the  skull,  for  in- 
stance, develops  by  several  cen- 
ters, it  is  to  be  regarded  as  formed 
by  the  fusion  of  several  primarily 
distinct  bones,  a  conclusion  which 
may  generally  be  confirmed  by  a 
comparison  of  the  bone  in  ques- 
tion with  its  homolQgues  iff  the 
th,«  r„i„  •     1.  .  '°*^''  ^^rtebrates.    Exceptions  to 

h.s  rule  occur  m  bones  situated  in  the  i^.edian  line  of  the  body 
these  occasionally  developing  from  two  centers  lying  one  on  ^^r 
s.de  of  the  median  line,  but  such  centers  are  usuaUy  to  b^  "  I^^' 
as  a  double  center  rather  than  as  two  distinct  centeranra^ 
merely  an  expression  of  the  fundamental  bilaterality  which  exist! 
even  m  median  structures. 

More  striking  exceptions  are  to  be  found  in  the  long  bones  in 

Sltrt^lttrt'''^  ^''"''''  ^""^  ^P-'^'  -^- which 
give  nse  to  the  e/nphyses  (F.g.  94,  ep,  ep'),  the  shaft  or  diaphysis  (d) 
bemg  formed  from  the  primary  center.     Similar  secondTcent  r 
appear  it,  marked  pftminences  on  bones  to  which  pOwerf^ks 


^JZlSi""^    OsSmCATION    ClNTM 

or  Fio.  93  Mom  Hiohly  Maonbied. 

e,  Ossifying  trabecule;  a,  cavity  of 

cartilage  network;  m,  marrow  cells;  «. 

poKMteal  bone;  fi,  irruption  of  peri- 

osteal  tissue;  pe,  periosteum.— (5.VW0- 


DEVKIOPMBNT  OF  BONE  ,.. 

they  do  no.  ap^fal'u^n'I^r  SrlTS 7  ''""'  ''^  '"^^  """ 
bone,  to  which  they  belong.    T^^  st^'  "'"''''  ""''"  °^  '"^ 
centers  g,ve  the  necessary  firmness  required 
for  articukr  surfaces  and  for  the  attachment 
of  muscles  and,  at  the  same  time,  make  pro- 
vision  for  the  growth  in  length  of  the  bone 
s.nce  a  plate  of  cartilage  always  intervenes 
between  the  epiphyses   and    the   diaphysis. 
Th,s  cartilage  continues  to  be  transformed 
mto  bone  on  both  its  surfaces  by  the  extension 
of  both  the  ep.phys.al  and  diaphysial  ossifica- 
K,n  mto  ,t,  and,  at  the  same  time,  it  grows 
m  thickness  w.th  equal  rapidity  untH  the 
bone  reaches  .ts  required  length,  whereupon 
the  rapidity  of  the  growth  of  the  cartikge 
dimimshes  and  it  graduaUy  becomes  com- 

The  growth  in  thickness  of  the  long  bones 

s.  however  an  entirely  different  process,  and 

■s  due  to  the  formation  of  new  layers  of  peri- 
osteal bone  on  the  outside  of  those  already 
present.  But  in  connection  with  this  process 
an  absorption  of  bone  also  takes  place  A 
section  through  the  middle  of  the  shaft  of  a 
humerus,  for  example,  at  an  early  stage  of 
development  would  show  a  peripheral  zone  of    v  '^■> 


Fro.  gi.—TsK  Osai- 
FICATON  C«Nra«s  Of 
TBZ  FxifVR. 

«.  and  4,  Secondary 
centers  for  the  great  and 
lesser  trochanters;  d 
diaphysis;  ef,  upper  and 
e£,  lower  epiphysis.— 
{Tarn.) 


iS8 


oxvxLomufT  or  lora 


Z      A  "I     ^^"T'**'  """*  '■'  '  «"dual  absorption  of  .he  .pongy 
bone  and  aUo  of  .he  cariier  layen,  of  perios.eal  bone,  .his  absoS 

H,lJr      T    ^  '■"*"  ""•'"■""''«=»'«•  "II*.  termed  osttoLls. 
•  ^Itl  !rT         T™"'  ""^^''^hyme.    By  .heir  ac.ion  the  bone 
«  enabled  .o  reach  ..s  requisi.e  diame.er  and  s.reng.h,  wiUiou. 
becoming  an  almos.  solid  and  unwieldy  mass  of  compa^.  tone. 
Duhng  the  ossification  of  the  cartilaginous  trabecute  os.eoblasU 

STfZ™  n  ^'  '"'  '^"^  *"•'*""'"'  ">*  "^•■"■"  '•»  which  Uiey 
he  forming  the  lac^  and  processes  radia.ing  ou.  from  .hem,  Ae 


TWO  MONTH,  *™.  r^  c««Tr  f;";^*^^  S'„^  <"  *  P«>  K.uxo 
Th,  he.,y  bl«k  l™  „pr«e„,.  ,h.  ^^„,  ton.  ,uii„^  by  u«  m«id«.-M/«a- 

coMi.  SO  charactensUc  of  bone  tissue.  In  U,e  growUi  of  peri- 
ostea^ bone  not  only  do  osteoblaste  become  enclosed,  but  blood- 
ITa  !^'  ?'  ^'«"'^^''  "»^  being  formed  in  this  way,  and 

substance  madder,  when  consumed  with  food,  tinites  the  boni.  Kp  nf 


DxnLonam  or  the  ikkuton  ,59 

«A^A  ,?•'•'?'»"•"*  »'  «»•  Skeleton. -Embryologically  con- 
sidered the  skeleton  is  composed  of  two  portions,  the  alial  skeU,^ 
conststrng  of  the  skull,  the  ver.eb«,  ribs,  and  sternum,  develop 


Fio.  96.-F«ONrAi  Section  ihmuoh  Mbodehmic  Somtu  of  a  Ca«  Fw..v„ 

"«,  Int.r«gm.„ul  .r.e,y;  my.  myotome;  .,  cenual  u„Z^Zc^«J^^, 

sc.  and  scp,  a„,„ior  „.|  posterior  porliot^XS'iSlW   '  ""'"' •""•' ■ 

from  the  sclerotomes  of  the  mesodermal  somites,  and  the  appen- 

he  bones  of  the  limbs,  and  which  arises  from  the  mesenchyme  of 
he  somattc  mesoderm.  It  will  be  convenient  to  consider  first  the 
development  of  the  axial  skeleton,  and  of  this  the  differentiation  of 
tne  vertebral  column  and  ribs  may  first  be  discussed. 


Ife 


DEVXLOniENT  OF  THE   VEKTHES 


i! 


nw  D«T«lopmtnt  of  th*  Vertebra  and  Wbi.-The  me«n- 
chyme  formed  from  the  sclerotome  of  each  meaodermic  lomite 
grow,  mward  toward  the  median -line  and  forms  a  mass  lying 
between  the  notochord  and  the  myotome,  separated  from  the 
similar  mass  in  front  and  behind  by  some  loose  tissue  in  which  lies 
an  mtencgmental  artery.    Toward  the  end  of  the  third  week  of 

development    the   cells   of   the 
posterior  portion  of  each  sclero- 
tome condense  to  a  tissue  more 
compact  than  that  of  the  anterior 
portion  (Fig.  96),  and  a  little 
kter  the  two  portions  become 
separated  by  a  cleft.    At  about 
the  same  time  the  posterior  por- 
tion sends  a  process  medially,  to 
enclose  the  notochord  by  uniting 
with    a    corresponding  process 
from  the  sclerotome  of  the  other 
side,  and  it  also  sends  a  pro- 
longation dorsally  between  the 
myotome  and  the  spinal  cord  to 
form  the  vertebral  arch,  and  a 
third  process  laterally  and  ven- 
..  ,  ,  trally  along  the  distal  border  of 

the  myotome  to  form  a  costal  process  (Fig.  97).  The  looser  tissue 
of  the  antenor  half  of  the  sclerotome  also  grows  mediaUy  to  sur- 
round the  notochord,  filbng  up  the  intervab  between  successive 
denser  portions,  and  it  forms  too  a  membrane  extending  between 
successive  vertebral  arches.  Later  the  tissue  surrounding  the  noto- 
chord, which  is  derived  from  the  anterior  half  of  the  sclerotome, 

associates.tself  with  the  posterior  portionof  the  preceding  sclerotome 
to  form  what  will  later  be  a  vertebra,  the  tissue  occupying  and 
adjacent  to  the  line  of  division  between  the  anterior  and  posterior 
portions  of  the  sclerotomes  condensing  to  form  intervertebral 
fibrocartilages.     Consequently  each  vertebra  is  formed  by  parts 


Fro.   97.— TsANSvxKi  SicnoN 

THSOCOH    THE    ImUVniUIAI.  PUIB 

o»  TH«  Fust  Cmviqu.  Vumu  of  a 
Cau  Eubiyo  or  8.8  im. 

be'.  Intervertebral  plate;  m',  fourth 
myotome;  j,  hypocbordal  bar;  XI,  ipinal 
accouty  nerve.— (Fror^.) 


DtmonaOT  of  the  vcitebu  ,«, 

the  myotome,     wi^.M.  H!^    '''"•^'*  •''""'^'^'=»  «"«™.,e  with 
0/  ./development  r  ^ vt^BrS  ""^ '"'  °'  •""''^-'^  -«' 

P«.ce«e.  of  the  a^SrTrvl^/ tru  ^.^  '  '  '    '^  '"  ""=  -"" 
line  below  the  notochoH   ,?»     ^T       ""«  """»  "-e  median 

and  come  into  contact  w^Tlfe  2  ''•"''^k  '.""'^"^  '"  '«»«* 
ligaments.     At  fi«t  tL  rib  c^^?  °"  •  °™'"« ''"' '^°^'°-^"'«''^''' 


i6a 


DXVXlOPUin  OV  THK  VXXTEBKS 


interspinous  ligaments  and  the  ligamenta  flava,  while  the  anterior 
and  posterior  longitudinal  ligaments  are  formed  from  unchondrified 
portions  of  the  tissue  surrounding  the  vertebral  bodies. 

As  was  pointed  out,  the  mesenchyme  in  the  region  of  the  cleft 
separating  the  anterior  and  posterior  portions  of  a  sclerotome  be- 
comes an  intervertebral  fibrocartilage,  and,  as  the  cartilaginous 
bodies  develop,  the  portions  of  the  notochord  enclosed  by  them 
become  constricted,  while  at  the  same  time  the  portions  in  the 
intervertebral  regions  increase  in  size.  Finally  the  notochord  dis- 
appears from  the  vertebral  regions,  although  a  canal,  representing 
its  former  position,  traverses  each  body  for  a  considerable  time,  but 
in  the  intervertebral  regions  it  persists  as  relatively  large  flat  disks 
forming  the  pulpy  nuclei  of  the  fibrocartilages. 

The  mode  of  developWnt  described  above  applies  to  the  great 
majority  of  the  vertebrse,  but  some  departures  from  it  occur,  and 
these  may  be  conveniently  consideiwl  before  passing  on  to  an 
account  of  the  ossification  of  the  cartilages.  The  variations  affect 
principally  the  extremes  of  the  series.  Thus  the  posterior  vertebra 
present  a  reduction  of  the  vertebral  arches,  tl;o^.-  of  the  last  sacral 
vertebrae  being  but  feebly  developed,  while  in  the  coccygeal  vertebne 
they  are  indicated  only  in  the  first.  In  the  first  cervical  vertebra, 
the  atlas,  the  reverse  is  the  case,  for  the  entire  adult  vertebra  is 
formed  from  the  posterior  portion  of  a  sclerotome,  its  lateral  masses 
and  posterior  arch  being  the  vertebral  arches,  while  its  anterior  aich 
is  the  hypochofdal  bar,  which  persists  in  this  vertebra  only.  A  well- 
developed  centrum  is  also  formed,  however  (Fig.  98),  but  it  does  not 
unite  with  the  parts  derived  from  the  preceding  sclerotome,  but 
during  its  ossification  unites  with  the  centrum  of  the  epistropheus 
(axis),  forming  the  odontoid  process  of  that  vertebra.  The  epistro- 
pheus consequently  is  formed  by  one  and  a  half  sclerotomes,  while 
but  half  a  one  constitutes  the  atlas. 

The  extent  to  which  the  ribs  are  developed  in  connection  with 
the  various  vertebne  also  varies  considerably.  Throughout  the  cer- 
vical region  they  are  short,  the  upper  five  or  six  being  no  longer  than 
the  transverse  processes,  with  the  tips  of  which  their  extremities 


MVEIOPIONT  OF  THE  VSBTIBM  ,63 


164 


DXVZLOFMKNT  OF  TBX  VERTEBRJC 


§ 


are  reduced  to  shoij  flat  plates,  which  unite  together  to  fonn  the 
lateral  masses  of  the  sacrum,  and,  finally,  in  the  coccygeal  r^n  the 
blaatemic  costal  processes  of  the  first  vertebra  unite  with  the  trans- 
verse processes  to  form  the  transverse  processes  of  the  adult  vertebra, 
but  no  indications  of  them  are  to  be  found  in  the  other  vertebrie 
beyond  the  blastemic  stage. 

The  third  stage  in  the  development  of  the  axial  skeleton  b^ins 
with  the  ossification  of  the  cartilages,  and  in  eachvertebra  there  are 
typically  as  many  primary  centers  of  ossification  as  there  wt  e 
originally  cartilages,  except  that  but  a  single  center  appears  in  the 
body.  Thus,  to  take  a  thoracic  vertebra  as  a  type,  a  center  appears 
in  each  half  of  each  vertebpl  arch  at  the  base  of  the  transverse  process 
and  gradually  extends  to  form  the  bony  lamina,  pedicle,  and  the 
greater  portion  of  the  transverse  and  spinous  processes;  a  single 
center  gives  rise  to  the  body  of  the  vertebra;  and  each  rib  ossifies 


Fio.  gg.—A,  A  Vestibia  at  Bimb;  B,  Lchbaii  Vkhtebiw  sbowikg  Sbconomtv 
Centzhs  of  Ossification. 
a  Center  for  the  articular  process;  c,  body;  tl,  kmKr  epiphysial  plale;  en.  upaer 
epiphysial  plate;  na,  vertebral  aith;  i,  center  for  spinous  process;  (,  center  for  irnwiiiim 
process.— (Sa^^.) 

from  a  single  center.  These  ■"arious  centers  appear  early  in  esdry- 
onic  life,  but  the  complete  transformation  of  the  cartil^es  into  base 
does  not  occur  until  some  time  after  birth,  each  vertebra  at  that 
period  consisting  of  three  parts,  a  body  and  two  halves  of  anarch, 
separated  by  unossified  cartilage  (Fig.  99,  A).  At  about  puberty 
secondary  centers  make  their  appearance;  one  appears  in  the  carti- 
lage which  still  covers  the  anterior  and  posterior  surt'aces  of  the 
vertebral  body,  producing  disks  of  bone  in  these  situations  (Fig.  99, 


DEVEIOPMENT  OF  IHE  VEKTEBM  jgj 

at  me  tips  of  the  articulating  processes.    The  epiphyses  so  f„rn^ 
remain  separate  until  growth  is  completed  and  be^wSe^Jr?? 
and  twenty-fifth  years  unite  with  the  bones  formS  fZ  H, 
centen,  which  have  fused  bv  thi.  ,i^.^   f  ^^  P'™*'^ 

Each  rib  ossifiL  frlm  .  5    .  '   **  ^°""  ^  ''"K'*^  ^«^««bra. 

—u        "° '^'"^  '"""  "  smgle  primary  center  situated  near  thr 
aa^e,  secondary  center,  appearing  for  .he  capitukm  and  tuS' 

centers  for  th^  ^^^rX'^J^7:SL:^'^  ^ 
mg  into  them  from  t,.e  vertebral  arch  centers^/?'  '^' 

tot  only  developing  a  separate  rib  center.    Furtherm^!^^' ^^ 


"'  aWitioc  to  the  tvpi^ircente^     vertebra  con^^Uy  po3^,i,^, 

s,coBdarv  r^.-^  -  Pi='''."°'««' «"«  'double)  otter  primarv  and  two 

'"      """■     *  »^  »«»i  reg-n  the  typ«l  centers  appear 


i  {J    I 


i66 


DEVELOTMENT   OF   THE   STERNUM 


in  all  five  vertebne,  with  the  exception  of  rib  centers  for  the  last  one 
or  two  (Fig.  wo)  and  two  additional  secondary  centers  give  rise  to 
plate-like  epiphyses  on  each  side,  the  upper  plates  forming  the 
articular  sarface  for  the  ilium.  At  about  the  twenty-fifth  year  all 
the  sacral  votetae  unite  to  form  a  single  bone,  and  a  similar  fusion 
occurs  also  in  the  rudimentary  vertebrae  of  the  coccyx. 

The  majority  of  the  anomalies  seen  in  the  vertebral  column  are  due 
to  the  imperfect  development  of  one  or  more  cartilages  or  of  the  centers  of 
onificatioB.  Thus,  a  failure  of  an  arch  to  unite  with  the  body  or  even  the 
coiq>lete  absence  of  an  arch  or  half  an  arch  may  occur,  and  in  cam  of 
spina  bifida  the  two  halves  of  the  arches  fail  to  unite  dorsaUy.  Occasion- 
ally the  two  parte  of  the  >louble  cartilaginous  center  for  the  body  fail  to 
unite,  a  doabie  body  resulting ;  or  one  of  the  two  parts  may  eadcely  fail, 
the  result  bcmg  the  formation  of  only  one-half  of  the  body  <il  dK  vertebra. 
Other  anoraafies  result  from  the  excessive  development  of  parts.  Thus, 
tfie  rib  of  the  sevesth  cervical  vertebra  may  sometimes  remain  distinct  and 
be  l<ng  enough  to  reach  the  sternum,  and  the  first  lumbar  rib  may  also 
fail  to  unite  with  its  vertebra.  On  the  other  hand,  the  first  thoracic  rib  is 
occasionally  found  Id  be  imperfect 


The  DcTVtofMaat  of  tte  Steianm. — Longitudinal  bars,  which 
are  formed  by  the  fusion  of  the  ventral  ends  of  the  anterior  eight  or 
nine  cartflagiaous  thoracic  ribs,  represent  the  futus  sternum.  At 
an  early  period  the  t>.o  bars  come  into  contact  anteriorty  and  iuse 
together  (Fig.  loi),  ami  at  tins  anterior  end  two  usually  indistinctly 
separated  masses  of  cartilage  are  to  be  observed  at  the  vidaity  of  tbf 
points  where  the  ventral  ends  of  the  cartilaginons  davides  anicalatc 
These  are  the  epislemal  cartilages  (em),  which  later  irmiHy  laite 
with  the  longitudinal  bars  and  form  part  of  the  maimbriian  sicrni 
thoti{^  occasionally  they  persist  and  ossify  to  form  the  ossa  snfrast^ 
nalia.  The  fusion  of  the  longitudinal  bars  gradually  extends  back 
ward  until  a  single  elongated  plate  of  cartilage  results,  with  wluch  tfc 
seven  anterior  ribs  are  united,  one  or  two  of  the  more  posteriw  ril» 
which  originally  took  part  in  the  formation  of  each  bar  havin; 
separated.  The  ponions  of  the  bars  formed  by  these  posterior  ribs 
constitute  the  xiphoid  process. 

The  ossification  of  the  sternum  (Fig.  102)  partakes  to  a  certaii 
extent  of  the  original  bilateral  segmental  origin  of  the  cartilag' , 


DEVZLOmZMT  OF   THE   STESNUM 


167 


but  there  is  a  marked  condensation  of  the  centers  of  ossification  and 
considerable  variation  in  their  number  also  occurs.  In  the  portion  of 
the  cartilage  which  lies  below  the  junction  of  the  third  rostal  cartilages 
a  senes  of  pairs  of  centers  appears  just  about  birth,  each  center 


F.G     iOl.-FoMMmm  OF  THE   STMNOII  o-  «  E»,RYO  OF   ABOUT  3  CM. 
el,  Climtk.  OK,  epkniiua  oBU^ie,— t«ii^.) 


P"»b»biy  representing  aa  epiphvsMj  center  #  a  carresponding  rib 
L^the  centers  «t  each  pair  h^  i«,d  the  single  centers  so  formed. 
-•xlBidiiig  through  the  tartilage.  evenoualfr  »««  t.  form  the  greater 
part  «f  the  body  of  the  bone.  In  .arh  of  Ae  two  uppermost  seg- 
menta  however,  but  a  single  center  ap,«.ars,  that  of  thr  -econd 
wgment  uniting  with  tiie  more  p<«terior  centers  and  forming  the 
.pper  part  of  the  body,  while  the  uppermost  center  gives  ri.,e  to  the 

nuuiubnum.  which  fc-  .sa"""*!" —.-..-  -^     i,  .      .  ; .     ,       , 

■"    ■  ""'•J  r-'— ■■  •*- =  'iicnci  mriir  unileo  toihe 
oooy  by  4  hinge-joini 


V\ 


Wk 


i68 


OeVElOniENT  OF 


SKULL 


A  failure  of  the  cartilaginous  bars  to  fuse  produces  the  condition 
known  as  cleft  sternum,  or  if  the  failure  to  fuse  affecU  only  a  pordon  of  the 
bare  there  results  a  perforated  sternum.  A  perforation  or  notching  of  the 
xiphoid  cartilage  is  of  frequent  occurrence  owing  lo  this  being  the  region 
where  the  fusion  of  the  bars  takes  place  last. 


Fio.     I03. — Sternuu    or 
Nkw-bokn    Child,   showing 
CXNTUS  or  Ossification. 
/  to  VII,  Cwtal  cartitages.— 
(GegenbaMr.) 


Fio. 


103. — RicoNSTKUcnoN  of  the  Chondro- 

CKAHIUli  of  an  EmitYO  OF  14  ] 


as,  AUsphenoid;  bo,  basiocripital;  bs,  bui- 
sphenoid;  eo,  exoccipital;  m,  Meckel's  cutikge; 
OS,  oibitcsphenoid;  j>,  periolic;  fs,  presphoioid; 
50,  sella  turcica;  s,  supraoccipital.— (Levi.) 


The  suprasternal  bones  are  the  rudiments  of  a  bone  or  cartilage,  the 
omostemum,  situated  in  front  of  the  manubrium  in  many  of  the  lower 
mammalia.  It  furnishes  the  articular  surfaces  for  the  clavicles  and  is 
possibly  formed  by  a  fusion  of  the  ventral  ends  of  the  cartilages  which 
represent  those  bones;  hence  ite  appearance  as  a  pair  of  bones  in  the  rudi- 
mentary condition. 

The  Development  of  the  Skull.— In  its  earliest  ,*ages  the 
human  skull  is  represented  by  a  continuous  mass  of  mesenchyme 
which  invests  the  anterior  portion  of  the  notochord  and  extends 
forward  beyond  its  extremity  into  the  nasal  region,  forming  a  con- 
fer the  nasal  process  (see  p.  99).  From  each  side  of  this  basal  mass 
a  wing  projects  dorsally  to  enclose  the  anterior  portion  of  the  medul 
lary  canal  which  will  later  become  the  cerebral  part  of  the  ceBtral 
nervous  system.     No  indications  of  a  segmental  origin  are  to  bi 


DEVELOPMENT  OF  THE   SKUU. 


169 


found  m  this  mesenchyme;  as  stated,  it  is  a  continuous  mass,  and 
this  IS  likewise  true  of  the  cartilage  which  later  develops  in  it 
The  chondrification  occurs  first  along  the  median  line  in  what 
wiU  be  the  occipital  and  sphenoidal  regions  of  the  skull  (Fir  ioO 
and  thence  gradually  extends  forward  into  the  ethmoidal  region  and 
to  a  certain  extent  dorsaUy  at  the  sides  and  behind  into  the  regions 
later  occupied  by  the  wiags  of  the  sphenoid  (as  and  os)  and  the 
squamous   portion   of  the  occipital  (.).    No  cartiUge   develops, 
however,  m  the  rest  of  the  sides  or  in  the  itx.f  of  the  skull,  but  the 
mesenchyme  of  these  regions  becomes  converted  into  a  dense  mem- 
brane of  comiective  tissue.    While  the  chondrification  is  proceeding 
m  the  regions  mentioned,  the   mesenchyme  which  encloses  the 
mtemal  ear  becomes  converted  into  cartilage,  forming  a  mass   the 
pcnoltc  capsuU  (Fig.  103,  p),  wedged  in  on  either  side  between  the 
occipital  and  sphenoidal  regions,  with  which  it  eventuaUy  unites  to 
form  a  continuous  chondrocranmm,  perforated  by  foramina  for  the 
passage  of  nerves  and  vessels. 

The  posterior  part  of  the  basilar  portion  of  the  occipital  cartilage 
presents  certain  peculiarities  of  development.     In   calf  embrvos 
Uiere  are  m  this  region,  in  very  e»ly  stages,  four  separate  condema- 
tions  of  mesoderm  corresponding  to  as  many  mesodermic  somites 
and  to  the  three  roots  of  the  hypo^ossal  nerve  together  with  the  first 
cervicalorsuboccipitalnerve  (Froriep)  (Fig.  104).     These mesenchv- 
ma^  masses   in   their  general   characters  and   relations  resemble 
vertebral  bodies,  and  there  are  good  reasons  for  believing  that  they 
represent  four  vertebrae  which,  in  later  stages,  are  taken  up  into  the 
skuU  region  and  fuse  with  the  primitive  chondrocranium      In  the 
human  embryo  they  are  iess  distinct  than  in  lower  mammals,  but  since 
a  three-rooted  hypoglossal  and  a  suboccipital  nerve  also  occur  in  man 
.t  IS  probable  that  the  corresponding  vertebne  are  also  represented, 
indeed,  confirmation  of  their  existence  may  be  found  in  the  fact 
that  during  the  cartilaginous  stage  of  the  skull  the  hypoglossal  fora- 
mina are  divided  into  three  portions  by  two  cartilaginous  partitions 
wnicn  sepai^tf*  the  *Ure^  -r*.-*-  -'-''_•-■ 

_.  ■    .         ''  "      "        '""'=  "■'  ;ii!;  "yiK>gi05Siii  iitTvu.     It  seems 
certain  from  the  evidence  derived  from  embryology  and  comparative 


»7o 


SEvxLOPimn  or  the  skuu 


•natomy  that  the  human  skull  is  composed  of  a  primitive  unseg- 
mental  chondrocranium  ptus  four  vertebra,  the  latter  being  added 

to  and  incorporated  with  the  occip- 
ital portion  of  the  chondrocranium. 
Emphasis  must  be  laid  upon 
the  fact  that  the  cartilaginous  por- 
tion of  the  skull  forms  only  the 
base  and  lower  portions  of  the  sides 
of  the  cranium,  its  entire  roof,  as 
well  as  the  face  region,  showing  no 
indication  of  cartilage,  the  mesen- 
chyme in  these  regions  being  con- 
verted into  fibrous  connective  tissue, 
which,  especially  in  the  cranial  re- 
gion, assumes  the  form  of  a  dense 
membrane. 

But  in  addition  to  the  chondro- 
cranium and  the  vertebra  incorpo- 
rated with  if,  other  cartilaginous  ele- 
ments enter  Into  the  composition  of 
the  skull.  The  mesenchyme  whic;. 
occupies  the  axis  of  each  branchial 
arch  undergoes  more  or  less  com- 
plete chondrification,  cartilaginous 
bars  being  so  formed,  certain  of 
which  enter  into  very  close  rela- 
tions with  the  skull.  It  has  been 
seen  (p.  92)  that  each  half  of  the 
first  arch  gives  rise  to  a  maxillary 
process  which  grows  forward  and 
ventrally  to  form  the  anterior 
boundary  of  the  mouth,  while  the 
remaining  portion  of  the  arch  forms 
the  mandibular  process.  The 
whole  of  the  axis  of  the  mandib- 


Fio-i««— FnoNTAi  Section 

inoiraB  TBI  OrcWlTAL  AND  Vmn 

Cmytcai  RagisM  at  a  Calf  Emmvo 

OF  8.7  IBL 

»"'■*  «',  Intervonehr*!  uuss 
w  ,  int  osrical  intentftikB^  ^ma 
i«.  mfaocdpHai  intcrvnvAnl  plate 
V~!lS!^  nerva;  ek,  notochord 
A,  vei  Icbiai  cMMnim;  w'~*  acciprtal 
uijuttJliMj;  m'—',  cenricsi  tiiiiMumq- 

^     .  n?o»i  ai  ^«wv-* 1  -  _  • 

juguiar  vrin ;  x  Bd'xi.  r«nis  Tnd  ipiuj 
•"■— " '    —nm.—ifr<ri^.) 


DEVXLOPHENT   Ot  TH«  8K0U,  jji 

uUr  process  becomes  chondrified,  forming  a  rod  known  as  UeckePs 
carlriage,  and  this,  at  its  dorsal  end,  comes  into  relation  with  the 
penouc  capsule,  as  does  also  the  dorsal  end  of  the  cartilage  of 
the  second  arch.  In  the  remaining  three  arches  cartilage  forms 
only  m  the  ventral  portions,  so  that  their  rods  do  not  come  into 
relation  w.th  the  skull,  though  it  wiU  be  convenient  to  consider 
their  further  history  together  with  that  of  the  other  branchial  arch 
cartilages.  The  -'rangement, 
of  these  cartilages  is  shonim  dia- 
grammatically  in  Fig.  105. 

By  the  ossification  of  these 
various  parts  three  categories  of 
bones  arc  formed:  (i)  cartilage 
bones  formed  in  the  chondro- 
cranium,  (2)  membrane  bones, 
and  (3)  cartilage  bones  devel- 
oping from  the  cartilages  of  the 
branchial  arches.     The  bones 
belonging  to  each  of  these  cate- 
gories are  primarily  quite  distort  from  one  another  and  from 
those  of  the  other  groups,  but  in  the  human  skull  a  very  consid- 
erable amount  of  fusion  of  the  primary  bones  takes'"place,'"and 
elemea.s  belonging  to  two  or  even  to  all  three  categories  may  unite 
to  form  a  single  bone  of  the  adult  skull.    In  a  certain  region  of  the 
chondrocranium  also  and  in  one  of  the  branchial  arches  the  original 
cartilage  bone  becomes  ensheathed  by  membrane  bone  and  event- 
uaUy  disappears  completely,  so  that  the  adult  bone,  although  repre- 
sented by  a  cartilage,  is  really  a  membrane  bone.    And.  indeed, 
this  process  has  proceeded  so  far  in  certain  portions  of  the  branchial 
arch  skeleton  that  the  original  cartikginous  representatives  are 
no  longer  developed,  but  the  bones  are  deposited  directly  in  connec- 
tive tissue.    These  various  modifications  interfere  greaUy  with  the 
precise  application  to  th.  human  skull  of  the  classification  of  bones 
rnto  the  three  categories  given  above,  and  indeed  the  tnie  significance 
of  certain  of  the  skull  bones  can  only  be  perceived  by  comparative 


Fig.  105.— Ducram  showing  the  Five* 

BiANCHiAL  Cartilages,  I  to  V. 

At,  Atlas;  Ax,  epistropheus;  3,  third 

cervital  vertebra. 


X7a 


oiaincAnoN  or  tke  cbondkocbanidm 


ttudies.  NeverthelcM  it  leeau  adviiable  to  retain  the  cUitificAtimi, 
indicating,  where  necessary,  the  confusion  of  bones  of  the  various 
categories. 

Th«  OMlflcaUon  of  the  Chondrocranlum.— The  ossification 
of  the  cartilage  of  the  occipital  region  results  in  the  formation  of 
four  distinct  bones  which  even  at  birth  are  separated  from  one 

another  by  bands  of  cartilage. 
The  portion  of  cartilage  lying  in 
front  of  the  foramen  magnum 
ossifies  to  form  a  basioccipital 
bone  (Fig.  io6,  bo),  the  portions 
on  either  side  of  this  give  rise  to 
the  two  exoccipUals  (eo),  which 
bear  the  condyles,  and  the  por- 
tion above  the  foramen  produces 

?»..._-'— Z<-^  *  w^oocei^/a;  (so),  which  repre- 

if    (fl».  P'a         s^«t»  *e  part  of  the  squamous 

/ill  )  /  J^        portion  of  the  adult  bone  lying 

'^i'K  ^^»)   \J^  below  the  superior  nuchal  line. 

^\/   W^\    ^-'^^  AH    that   portion   of  the  bone 

which  lies  above  that  line  is 
composed  of  membrane  bone 
which  owes  its  origin  to  the 
fusion  of  two  or  sometimes  four 
centers  of  ossification,  appearing 
m  the  membranous  roof  of  the  embryonic  skuU.  The  bone  so 
formed  {if)  represents  the  mtffparieba  of  lower  vertebrates  and,  at 
an  early  stage,  unites  with  the  supraoccipital,  although  even  at 
burth  an  indication  of  the  line  of  union  of  the  two  parts  is  to  be  seen 
m  two  deep  incisions  at  the  sides  of  the  bone.  The  union  of  the 
exoccipitals  and  supraoccipital  takes  place  in  the  course  of  the  first 
or  second  year  after  birth,  but  the  basioccipital  does  not  fuse  with 
the  rest  of  the  bone  until  the  sixth  or  eighth  year.  It  will  be  noticed 
that  no  special  centers  occur  for  the  four  occipital  vertebra,  these 
structures  ha\ing  become  completely  incorporated  in  the  chondro- 


©.; 


Ro.  106.— OconTAL  BoNi  or  a  Frrns 

AT  TBXH. 

bo,  Basioccipital;  w,  exoccipital;  ip,  in- 
terparietal; so,  supraoccipital. 


omincAnoN  of  thb  chondiociamium 


173 


crwiium,  and  even  the  cartiUginous  parUtions  which  divide  the 
hypogloMal  foramina  usually  disappear  during  the  process  of 
ossification. 

Two  pairs  of  centers  have  been  described  for  the  interparietal 
bone  and  it  has  been  claimed  that  the  deep  Uteral  incisions  divide 
the  lower  pair,  so  that  when  the  incisions  meet  and  persist  as  the 
suiwa  mendosa,  separating  the  so-called  inca  bone  from  the  rest  of 
the  occipital,  the  division  does  not  correspond  to  the  line  between 
&e  supraoccipital  and  the  interparietal,  but  a  portion  of  the  latter 
bone  remains  in  connection  with  the  supraoccipital.  Mall,  how- 
ever, in  twenty  preparations,  found  but  a  single  pair  of  centers  for 
the  interparietal. 

Occasionally  an  additional  pair  of  small  centers  appear  for  the 
uppermost  angle  of  the  interparietal,  and  the  bones  formed  from 
them  may  remain  distinct  as  what  have  been  termed  fontaneUe 
hones. 


Fio.  107.— Sfbtmoid  Boni  noii  Ehbkyo  or  3}  to  4  Months. 
The  puts  wWdl  «re  stiU  cartilaginouj  ue  represented  in  black.    <u  AloDhenoid- 
h,  bubphenold;  /,  lingul.;  „,  orbito.phenoid ;  /intemiU  pterygoid  pUii!-^^.) 

In  the  sphenoidal  region  the  number  of  distinct  bones  which 
develop  is  much  greater  than  in  the  occipital  region.  At  the  begin- 
ning of  the  second  month  a  center  appears  in  each  of  tiie  cartilages 
which  represent  the  alispkenoids  (great  wings).  These  cartilages 
do  not,  however,  represent  the  entire  extent  of  the  great  wings  and 
their  ossification  gives  rise  only  to  those  portions  of  the  bone  in  the 
neighborhood  of  the  foramina  ovale  and  rotundum  and  to  the 
lateral  pterygoid  plates.  The  remaining  portions  of  the  wings,  the 
orbital  and  temporal  portions,  develop  as  membrane  bone  (Fawcett) 


mCMOCBTf   nSOUITION  TBI  CHAIT 

(ANSI  and  ISO  TEST  CHADT  No.  2) 


III  i-o 

lii|2£     |ZS 

11111^^= 

la  la    [2.2 

I:^  lis  ■" 

1  '*' 

■  1.8 

i^ll^l^ 


^ 


APPLIED  IIVMGE     In 

ieS3  Eait  Main  5tr«el 

Roctxslaf.  N«w  York        U609       USA 

(716)   482  -  0300  -  PfiOfw 

(716)  2111!-  59S9  -Fa. 


174 


OSSIFICATION   OF  THE  CRONDSOCRANICU 


and  early  unite  with  the  portions  formed  from  the  cartilage.  At 
the  end  of  the  second  month  a  center  appears  in  each  orbilosphenoid 
(lesser  wing)  cartilage  (Fig.  107,  os),  and  a  little  later  a  pair  of 
centers  (i),  placed  side  by  side,  are  developed  in  the  cartilage 
representing  the  posterior  portion  of  the  body;  together  these  form 
what  is  known  as  the  basispkenoid.  Still  later  a  center  appears  on 
either  side  of  the  basisphenoids  to  form  the  linguUe  (I),  and  another 
pair  appears  in  the  anterior  part  of  the  cartilage,  between  the  orbito- 
sphenoids,  and  represent  the  prespkenoid. 

In  addition  to  these  ten  centers  in  cartilage  and  the  membrane 
portion  of  the  alisphenoid,  two  other  membrane  bones  are  included 
in  the  adult  sphenoid.  Thus,  a  little  before  the  appearance  of  the 
center  for  the  alisphenoids  ^n  ossification  is  formed  in  the  mesen- 
chyme of  each  lateral  wall  of  the  posterior  part  of  the  nasal  cavity 
and  gives  rise  to  the  medial  lamina  of  the  pterygoid  process,  the 
mesenchyme  at  the  tip  of  the  ossification  condensing  to  form  a 
cartilaginous  hook-like  structure  over  which  the  tendon  of  the  tensor 
veli  palatini  plays.  This  cartilage  later  ossifies  to  form  the  pterygoid 
hamulus,  the  medial  pterygoid  lamina  being  thus  a  combination  of 
membrane  and  cartilage,  the  latter,  however,  being  a  secondary 
development  and  quite  independent  of  the  chondrocranium. 

By  the  sixth  month  the  Unguis  have  fused  with  the  basisphenoid 
and  the  orbitosphenoids  with  the  presphenoid,  and  a  little  later  the 
basisphenoid  and  presphenoid  unite.  The  alisphenoids  and  medial 
pterygoid  laminae  remain  separate,  however,  until  after  birth,  fusing 
with  the  remaining  portions  of  the  adult  bone  during  the  first  year. 

The  cartilage  of  the  ethmoidal  region  of  the  chondrocranium 
forms  somewhat  later  than  the  other  portions  and  consists  at  first 
of  a  stout  median  mass  projecting  downward  and  forward  into  the 
nasal  process  (Fig.  108,  Ip),  and  two  lateral  masses  (Im),  situated  one 
ou  either  side  in  the  mesenchyme  on  the  outer  side  of  each  olfactory 
pit.  Ossification  of  the  lateral  masses  or  cclelhmoids  begins  rela- 
tively early,  but  it  appears  in  the  upper  part  of  the  median  cartilage 
only  after  birth,  producing  the  crista  galli  and  the  perpendi'-alar 
plate,  which  together  form  what  is  termed  the  mesethmoid.    When 


OSSmCATION  OF   THE   CHONDROCBANIDM 


175 


first  formed,  the  three  cartilages  are  quite  separate  from  one  another, 
the  olfactory  and  nasal  nerves  passing  down  between  them  to  the 
olfactory  pit,  but  later  trabecule  begin  to  extend  across  from 
the  mesethmoid  to  the  upper  part  of  the  ectethmoids  and  eventually 
form  a  fenestrated  horizontal  lamella  which  ossifies  to  form  the 
cribriform  plate. 

The  lower  part  of  the  median  cartilage  does  not  ossify,  but  a 

center  appears  on  each  side  of  the  median  line  in  the  mesenchyme 

behind  and  below  its  posterior  or  lower 

border.  From  these  centers  two  verti- 
cal bony  plates  develop  which  unite 

by  their  median  surfaces  below,  and 

above  invest  the  lower  border  of  the 

cartilage  and  form  the  vomer.     The 

portion  of  the  cartilage  which  is  thus 

invested  undergoes  resorption,  but  the 

more  anterior  portions  persist  to  form 

the  cartilaginous  septum  of  the  nose. 

The  vomer,  consequently,  is  not  really 

a  portion  of  the  chondrocranium,  but 

IS   a   membrane  bone;    its    intimate 

relations  with  the  median  ethmoidal 
cartilage,  however,  make  it  convenient 
to  consider  it  in  this  place. 

When  first  formed,  the  ectethmoids  arc  masses  of  spongy  bone 
and  show  no  indication  of  the  honeycombed  appearance  which  they 
present  in  the  adult  skull.  This  condition  is  produced  by  the 
absorption  of  the  bone  of  each  mass  by  evaginations  into  it  of  the 
mucous  membrane  lining  the  nasal  cavity.  This  same  process  also 
brings  about  the  formation  of  the  curved  plates  of  bone  which 
project  from  the  inner  surfaces  of  the  lateral  masses  and  are  known 
as  the  superior  and  middle  concha:  (turbinated  bones).  The  inferior 
and  sphenoidal  conchse  are  developed  from  special  centers,  but 
belong  to  the  same  category  as  the  others,  being  formed  from  por- 
tions  of  the  lateral  ethmoidal  cartilages  which  become  almost 


Fig.  108.— Anterior  Portion 
OF  THE  Base  of  the  Skuli.  of  a 
6  TO  7  XfoNTHS'  Embryo, 

The  sh.'iilt'(l  parts  represent 
cartilage,  eft,  Cribriform  plate; 
Im,  lateral  mass  of  the  ethmoid; 
tp,  perpendicular  plate;  of,  optic 
foramen;  oj,  orbitosphcnoid. — 
{Afirr  von  Spee.) 


i 


■ 


'M 


176 


OSSIFICATION   OP   THE   CHONDROCRANIUM 


separated  at  an  early  stage  before  the  ossification  has  made  much 
progress.  Absorption  of  the  body  of  the  sphenoid  bone  to  form 
the  sphenoidal  cells,  of  the  frontal  to  form  the  frontal  sinuses,  and 
of  the  mamillaries  to  form  the  maxillary  antra  is  also  produced  by 
dUtgrowths  of  the  nasal  mucous  membrane,  all  these  cavities,  as 
well  as  the  ethmoidal  cells,  being  continuous  with  the  nasal  cavities 
and  lined  with  an  epithelium  which  is  continuous  with  the  mucous 
membrane  of  the  nose. 

^,\l  the  lower  mammaUa  the  erosion  of  the  mesial  surface  of  the 
ectethmoidal  cartilages  results,  as  a  rule,  in  the  formation  of  five  conchae, 
wmie  in  man  but  three  are  usually  recognized.  Not  infrequenUy 
^ZT  ^j?"."^""  .-"^ddle  concha  shows' indications,  moreTr  les 

SMnd  »;  ,l^^-T°V/°  ^  uPP"  *"'*  *  '°"'"  P°'"°"'  '^Wch  corre- 
spond to  the  third  and  fourth  bones  of  the  typical  mammaUan  arrange- 
ment Furthermore,  at  tne  upper  portion  of  the  nasal  wall,  in  front  of 
the  superior  concha,  a.  slight  elevation, 
termed  the  agger  nasi,  is  always  observa- 
ble, its  lower  edge  being  prolonged  down- 
ward to  form  what  is  termed  the  uncinate 
process  of  die  ethmoid.  This  process 
and  die  agger  togeUier  represent  the  up- 
permost concha  of  the  typical  arrange- 
ment, to  which,  therefore,  die  human 
arrangement  may  be  reduced. 

A  number  of  centers  of  ossifica- 
tion— the  exact  number  is  yet  uncer- 
tain—appear in  the  periotic  capsule 
during  the  later  portions  of  the  fifth 
month,  and  daring  the  sixth  month 
these  unite  together  to  form  a  single 
center  from  which  the  complete  ossi- 
fication of  the  cartilage  proceeds  to  form  the  petrous  and  mastoid 
portions  of  the  temporal  bone  (Fig.  109,  p).  The  mastoid  process 
does  not  reaUy  form  until  several  years  after  birth,  being  produced 
by  the  hollowing  and  bulging  out  of  a  portion  of  the  petrous  bone 
by  out-growths  from  the  lining  membrane  of  the  middle  ear  The 
cavities  so  formed  are  the  mastoid  cells,  and  their  relations  to  the 
middle-ear  cavity  are  in  all  respects  similar  to  those  of  the  ethmoidal 


Fig.    log.— The    Temposai. 
Bone  at  Busth.    The  Styloid 
Process  and  AuDrroKY  Ossicles 
ARE  NOT  Represented. 
p.  Petrous  bone;  s,  squamosal; 
I,  tympanic. — (Poiricr.) 


OSSIFICATION   OP   THE   CHONDROCRANIITM  177 

and  sphenoidal  cells  to  the  nasal  cavities.  The  remaining  portions 
of  the  temporal  bone  are  partly  formed  by  membrane  bone  and 
partly  from  the  branchial  arch  skeleton.  An  ossification  appears  at 
the  close  of  the  eighth  week  in  the  membrane  which  forms  the  side 
of  the  skull  in  tho  temporal  region  and  gives  rise  to  a  squamosal 
bone  (s),  which  later  unites  with  the  petrous  to  form  the  squamosal 
portion  of  the  adult  temporal,  and  another  membrane  bone  the 
tympamc  (/),  develops  from  a  center  appearing  in  the  mesenchyme 
surrounding  the  external  auditory  meatus,  and  later  also  fuses  with 
the  petrous  to  form  the  floor  and  sides  of  the  ertemal  meatus,  giving 
attachment  at  its  inner  edge  to  the  tympanic  membrane.  Finally 
the  styloid  process  is  developed  from  the  upper  part  of  the  second 
branchial  arch,  whose  history  will  be  considered  later. 

The  various  ossifications  which  form  in  the  chondrocranium  and 
the  portions  of  the  adult  skull  which  represent  them  are  showr  .  :he 
following  table : 


Region  of 
Chondrocranium. 


Ossification. 


1  fiasioccipital 

C»ccipital J  Exoccipitals 

Supraoccipital 


Sphenoidal 


Ethmoidal  , 


Basisphenoid 

Presphenoid 

Lingulae 

Alisphenoids 

Orbitosphenoids 

Mesethmoid 


Ectethmoids 

Inferior  concha. 
Sphenoidal  concha. 


Parts  of  Adult  Skull; 

Basilar  process. 
Condyle* 

Squaraoi  ilon  h;  low  superior  nuchal 

lire. 

Body. 

Greater  wings  (in  part). 
Lesser  wings. 
Lamina  perpendli  ularis. 
Crista  galU. 
Nasal  septum. 
Lateral  masses. 
Superior  concha. 
Middle  concha. 


I'eriolic  capsule /  Mastoid. 

\  Petrous. 

The  Membrane  Bones  of  the  Skull.-In  the  membrane  form- 
mg  the  sides  and  roof  of  the  skull  in  the  second  stage  of  its  develop- 


»78  ia£  tmaum  bonis  o»  ihe  Bxxnx 

mem  <H.sifications  appear,  which  give  rise,  in  addition  ,o  the  inter- 
pane^al  and  squamosal  bonesalready  n.entioned  in  connection  wUh 

om^r  h      "h   T^?''  '°  '""^  ^""^'  ''"'»>-"^-    E-ch  of  the 
nToT.h        .  r^'T  ''"■"  "  "°«'^  "»'<='  ^'"'■'^h  appears  at  the 

Ume  from  two  centers  situated  symmetricaUy  on  each  side  of  the 
median  hne  and  eventuaUy  fusing  completely  to  form  a  single  bonl 
a^Aough  more  or  less  distinct  indications  of  a  median  suture  A 
metopic,  are  not  infrequently  present. 

Furthermore    ossifications  appear  in  the  mesenchyme  of  the 

S?hic?r  'f  '°™  "f  '"^'^'  ''""''^-  -•»  ^^^---'^  bones,  aU 
of  which  anse  from  smgle  centers  of  ossification.  In  tf.e  case  of  each 
zygomatic  bone,  however,  th«e  osseous  thickenings  ap^S  onThe 

he  thick  n"  "'  ""''"'''  °^''""°°'  ^'^'^'^ '"-  di-PPears  and 
the  thickenings  unite  to  form  the  adult  bone,  though  occ^naUv 

strirTlt'/T'''  **"'''  ^^  "'''"''y  ^"^  ''^""«1'  belongs  also 
shictly  to  the  cat^ory  of  membrane  bones,  as  do  also  th.Taxilte 

bran  h   T    f '^""^  '^""'  '"'^'''''  P^^^rily  telon,ing  to  Z 
branchial  arch  skeleton,  with  which  they  will  be  considen^d" 

The  purely  membrane  bones  in  the  skull,  are,  then,  the  following: 

Pterygoid..  ™»f  «)>■»»««>  porton  of  ocdpitaL 

SquaS«.U MBl«lp,.q,g„idpUtei. 

tympMic SquMwut  poi&m  oJ  temporata. 

Pirirtals.    ^ympwiKptatei  of  temporals. 

FrontaL 

Nasals. 

Lachrymals. 

Zjrgomadcs, 

Vomer. 

be.n"l°''f"'"*'°"  "*  *^*  Branchial  Arch  Skeleton.-It  has 
S^nd-rr  ^^-  '''^  "'^'  "  cartilaginous  bar  develops  only  in  the 

process  no  cartilaginous  skeleton  fonns,  but  two  membrane  bones. 


ossmcAnoH  of  bbanchial  a«ch  skieton  x;, 

Jem^r  ""\"T^'  "«  d*-e>°Ped  in  it.  their  cartilaginous 
representatives  which  are  to  be  found  in  lower  vertebrates  TvT 
been  suppressed  by  a  condensation  of  the  development  Thf 
t^tme  bone  develops  from  a  single  center  of  ossification  but  for 
One  oT^"  no  less  than  five  centers  have  been  described^  rl) 
a?contaShf ,"  ""■:  '"r"'  °'  *=  ^'^^"^  •x'^'l"  "i  th*  bone 

;  ^eTrd  t tTf  r'  r""'  ''"'^'  '^  ^°"d  '0™^  'he  nasal 
process  and  the  part  of  the  alveolar 

border  which  contains  the  canine 
tooth;  a  third  the  portion  which  con- 
tains the  incisor  teeth;  while  the 
fourth  and  fifth  centers  lie  above  the 
first  and  give  rise  to  the  inner  and 
outer  portions  of  the  orbital  plate 
and  the  body  of  the  bone.  The 
first,  second,  fourth,  and  fifth  por- 
tions early  unite  together,  but  the 
third  center,  which  really  lies  in  the 

venteal  part  of  the  nasal  process,  remains  separate  for  some  time 
fyrming  what  .s  termed  the  pr.ma:^,  a  bone  which  remains  ri 
manently  distinct  in  the  majority  of  the  lower  mammals. 

the^S^''*M.il"'hr"'"''  ^"PK"«»'  "to  the  development  of 
osSoTon^efvW  Z',:;'  T^'^y^'  it  ha,  but  two  center,  o 
the  bone^  The  S/.Tr^,?''  P'f°«^  ""d  t^e  other  to  the  rest  of 
of  the  skth  wedc^^^     ^'  "^^  ""  appearance  about  Uie  middle 

mJfeJ^t" liteT  'f^\^^^^  from  a  failure  of  the 
D  lo^nrf  V^  to  uwte  completely  with  the  frontonasal  process  fsee 

their  un,on(ie  aOso  p  ,^{  ^'^  '^'^''"  '^^  '"°  *~"«»  ="•<»  P«vem 

bonrtdT"'f°''!r'''''''^'''^«'=P*^^'«=*^«^" 'he  tympanic 

1  thecaS  r~H-'^f '"  *'  '"""'"''"  ^^^'y  "^  ""^  '"■'  ">'•'  PO«io» 
bones  of  the  middle  ear,  the  malleus  and  incus,  a  description  of 


FiC.  I  lO.— DlAOHAK  OF  THl  OSSI- 
FICATIONS OF  WHICH  TBI  Maxilla 

IS  COMPMED,  AS  SMN  FIOll  THE 
OUTKR  SUSFACI.  TBI  A«»OW 
fASSES     THKOUGH     THE     iHnuOI- 

BiTAi   Canal.— (From   „„   spee 


T 


180 


OSSIFICATION  OP  BRANCHIAL  A«CH  SKELETON 


Whose  further  development  may  be  postpoaed  until  a  later  chapter 
At  about  the  middle  of  the  sixth  week  of  development  a  plate  of 
membrane  bone  appears  to  the  outer  side  of  the  lower  portion  of  the 
cartiUge  and  gradually  extends  to  form  the  body  and  ramus  of  the 
mandible. 

In  the  region  of  the  body  the  bone  develops  so  as  to  enclose  the 
cartilage,  together  with  the  inferior  alveolar  (dental)  nerve  which 
hes  to  the  outer  side  of  the  cartilage,  but  in  the  region  of  the  ramus 


CAT 


F.O.  i.i.-Monn,  of  Rioht  Hah  of  Mandibix  of  a  Fetus  95  «,.  «  Unoth: 

SIEK  FIOK  THE  MESIAI  SoiFACE.  "-"NOTH, 

the  bone  remains  entirely  to  the  outer  side  of  the  cartilage  and  nerve 
whence  the  position  of  the  mandibular  foramen  on  the  inner  surface 
of  the  adult  bone.  The  anterior  portion  of  Meckel's  cartilage 
becomes  ossified  by  the  extension  of  ossification  from  the  membrane 
bone  into  it,  the  portion  corresponding  to  the  body  of  the  bone  behind 
the  mental  foramen  disappears  and  the  portion  above  the  mandibu- 
lar foramen  is  said  to  become  transformed  into  fibrous  connective 
tissue  and  to  persist  as  the  spheno-mandibular  ligament.  At  the 
upper  extremity  of  the  ramus  two  nodules  of  cartilage  develop,  quite 
independently,  however,  of  Meckel's  cartilage  (Fig.  in,  Cr  and  Cy) 


OSSmCATION  OF  BRANCHUl  ARCH  SKIEION  igj 

Md  ossification  extends  into  these  from  fi.e  ramus  to  form  the 
coronoid  and  condyloid  processes.  And,  finally,  two  other  de- 
pendent cartilages  appear  toward  the  anterior  extr  mity  o  elh  ha  f 


F,0.  .„.-DlAC.AM  SBOWmo  T^  CArEfio.^  TO  WHICH  THE  BONE,  OT  THE  SkuXL 

spheBoid;  £^;exiSpS-  ^  fronSSSv  WnS'*??'!^  arch  elements!^  AS,  Ali- 
.^fn,  mandible;  Vx,  miiSi  T^ir^l',.?''' S-S  T'^.-^S^'  ^-  "yson^lic; 
5,,  s,».mosal;5,,  styloid  pticess;' nf^'Al^S^iag^:  ^^".J'S^Sc'-P'^^P"*'' 

bLd^^°r  Z  ^'•*'''^f'^«""  (C.)  -d  the  other  at  the  lower 
!!.^r.^f'^S'-"?  •  -  *?°  "'  '""'  ■"'=°n«''ated  into  the  bone 
without  developing  special  centers  of  ossification. 


In 


;''!| 


i8a 


OSSlMCAnON  OF  BBANCHUL  ARCH  SKEIETON 


Each  half  of  the  mandible  thus  ossifies  from  a  single  center,  and 
is  essentially  a  membrane  bone  replacing  a  cartilaginous  precursor. 
At  birth  the  two  halves  are  united  at  the  symphysis  by  fibrous  tissue, 
into  which  ossification  extends  later,  union  occurring  in  the  first 
or  second  year. 

T'le  upper  part  of  the  cartilage  of  the  second  branchial  arch  also 
comes  into  relation  with  the  tympanic  cavity  and  ossifies  to  form  the 
styloid  process  of  the  temporal  bone.  The  succeeding  moiety  of  the 
cartilage  undergoes  degeneration  to  form  the  stylohyoid  ligament, 
while  its  most  ventral  portion  ossifies  as  the  lesser  comu  of  the  hyoid 
bone.  The  great  variability  which  may  be  observed  in  the  length 
of  the  styloid  processes  and  of  |he  lesser  comua  of  the  hyoid  depends 
upon  the  extent  to  which  the  ossification  of  the  original  cartilage 
proceeds,  the  length  of  the  stylohyoid  ligaments  being  in  inverse 
ratio  to  the  length  of  the  processes  or  comua.  The  greater  comua 
of  the  hyoid  are  formed  by  the  ossification  of  the  cartilages  of  the 
third  arch,  and  the  body  of  the  bone  is  formed  from  a  cartilaginous 
plate,  the  copula,  which  unites  the  ventral  ends  of  the  two  arches 
concerned. 

Finally,  the  cartilages  of  the  fourth  and  fifth  branchial  arches 
early  fuse  together  to  form  a  plate  of  cartilage,  and  the  two  plates 
of  opposite  sides  unite  by  their  ventral  edges  to  form  the  thyreoid 
cartilage  of  the  larynx. 

The  accompanying  diagram  (Fig.  iia)  shows  the  various  struc 
tures  derived  from  the  branchial  arch  skeleton,  as  well  as  some  of 
the  other  elements  61  the  skull,  and  a  r&um<  of  the  fate  of  the  bran 
chial  arches  may  be  stated  in  tabular  form  as  follows,  the  parts  repre- 
sented by  cartilage  which  becomes  replaced  by  membrane  bone 
being  printed  in  italics,  while  membrane  bones  which  have  no 
cartilaginous  representatives  are  enclosed  in  brackets: 


istuA. 


[  (Mazilk). 
(Palatine). 
I  Mallena. 
J  lacuB. 

Spheno-mandibtUar  ligament 

I  MMlRMf. 


idanh. 


DEVEIOPMENT   OF  APPENDICDIAR   SKELETON  183 

Slyloiil  [iniicss  of  ihr  It-mporal. 

Slylr  ^-v  ,1,1  tiganu-nl. 

Lcswr.    rnunfhyuM. 

5''  *"'' Grraltr  lornu  nl  hyoiil. 

4th  and  5th  nithw Thyreoid  rarliKf  o(  larym. 

The  Development  of  the  Appendicular  Skeleton.— While 
the  greater  portion  of  the  axial  skeleton  is  formed  from  the  sclero- 
tomes of  the  mesodermic  somites,  the  appendicular  skeleton  is  ! ;  ,i(|| 
derived  from  the  somatic  mesenchyme,  which  is  not  divided  into  I  i  iF 
metamcrcs.  This  mesenchyme  forms  th;  core  of  the  limb  ( udand 
becomes  converted  into  cartilage,  by  the  ossification  of  which  all  the 
bones  of  the  limbs,  with  the  possible  exception  of  the  clavicle,  arc 
formed. 

Of  the  bones  of  the  pectoral  girdle  the  clavicle  requires  further 
study  before  it  can  be  certain  whether  it  is  to  be  regarded  as  a  purely 
cartilage  bone  or  as  a  combination  of  cartilage  and  membrane 
ossification  fOegenbaur).  It  is  the  first  bone  of  the  skeleton  to 
ossify,  two  centers  appearing  for  each  bone  at  about  the  sixth  week 
of  development.  The  tissue  in  which  the  ossifications  form  has 
certain  peculiar  characters,  and  it  is  difficult  to  say  whether  it  is  to  be 
regarded  as  cartilage  which,  on  account  of  the  early  differentiation 
of  the  center,  has  not  yet  become  thoroughly  differentiated  histologic- 
ally, or  as  some  other  form  of  connective  tissue.  However  that  may 
be,  true  cartilage  develops  on  cither  side  of  the  ossifying  region,  and 
into  this  the  ossification  gradually  extends,  so  that  at  least  a  portion 
of  the  bone  is  preformed  in  cartilage. 

The  scapula  is  at  first  a  single  pla..  of  cartilage  in  which  two 
centers  of  ossification  appear.    One  of  these  gives  rise  to  the  bod 
and  the  spine,  while  the  other  produces  the  coracoid  process  (Fig. 
1 13,  CO),  the  rudimentary  representative  of  the  coracoid  bone  which 
extends  between  the  scapula  and  sternum  in  the  lower  vertebrates.  -  .• 

The  coracoid  does  not  unite  with  the  body  until  about  the  fifteenth  ||j 

year,  and  secondary  centers  which  give  rise  to  the  verteb.  al  edge  (6)  ^'^ 

and  inferior  angle  of  the  bone  (a)  and  to  the  acromion  process  (c) 
unite  with  the  rest  of  the  bone  at  about  the  twentieth  year. 


lS4 


DlVElomiNT  OF  AWENDICUUt  SMIKON 


ep-physes  w..h  the  shaft  uking  p,ace  between  the  seve„ren7hTnd 


Fio.  113.— Thb  Ossification  Cin- 
Ti«s  o»  TH«  Scapula. 
a,  »,  ud  c,  Secondary  centm  for 
Uk  angle,  vertebral  border,  and  aero- 

e™°!L'(r;,/S5'"'""-™^-'"<'p'- 


Fio.  114.— RicoNSTuncriOH  of  an 
Ekskonic  Caipus. 

e,  Centrale;  «,  triquetral;  /»,  lunate- 
«,  capiute;  ^,  pisiform;  jc,  navicular;  (' 
8"jJJ^multangular;  <r,  lesser  multaoguir;' 


twentieth  years     The  radius  and  ulm  each  possesses  a  single  epi- 

for    he  shaft,    he  proximal  epiphysial  center  for  the  ulna  giving 
rise  to  the  tip  of  the  olecranon  process.  '^ 

The  embryological  development  of   the  carpus  is  somewhat 
complicated.    A  cartilage  is  found  representing  each  of  Tb^ne 
normally  occurring  in  th.  aa  It  (Fig,  rr4),  and'these  a  e  aLng^d 
.n  two.distmct  r^ws.  a  proxi.ial  one  consisting  pf  three  elemeT 


DEVEtOPMENT   Or  APPENDICITLAK   SEELETON  185 

named  from  their  relation  lo  the  bones  of  the  forearm,  radiaU 
tntermedmm,  and  ulnare;  and  a  distel  one  composed  of  four  elements' 
termed  carfalia.    In  addition,  a  cartilage,  termed  the  /nsi/orm  is 
found  on  the  ulnar  side  of  the  proximal  row  and  is  generally  regarded 
as  a  sesamoid  cartiUgc  developed  in  the  tendon  of  the  flexor  carpi 
ulnaris,  and  furthermore  a  number  of  inconstant  cartilages  have 
been  observed  whose  significance  in  the  majority  of  cases  is  mort  or 
less  obscure.    These  accessory  cartilages  either  disappear  in  later 
stages  of  development  or  fuse  with  neighboring  cartilages,  or,  in  rare 
cases,  ossify  and  form  distinct  elements  of  the  carpus.    One  of  them 
however,  occurs  so  frequently  as  almost  to  deserve  classification  a.' 
a  constant  element;  it  is  known  as  the  centrate  (Fig    ,14   c)  and 
occupies  a  position  between  the  cartilages  of  the  proxinal  and  distal 
rows  and  apparently  corresponds  to  a  cartilage  ty     ally  present 
in  lower  forms  and  ossifying  to  form  a  distinct  bone,    in  the  human 
carpus  Its  fate  varies,  as  it  may  either  disappear  or  unite  wth  ether 
cartiUges,  that  with  which  it  most  usually  fuses  being  probably  the 
radiale.    There  is  evidence  also  to  show  that  another  of  the  accessor, 
cartilages  unites  with  the  ulnar  element  of  the  distal  row,  represent- 
mg  the  carpale  v  typically  present  in  lower  forms. 

Each  of  the  elements  corresponding  to  an  adult  bone  ossifies 
from  a  single  center  with  the  exception  of  carpale  iT^  which  has  two 
centers,  a  further  indication  of  its  composite  character.  The  rela- 
tion of  the  cartilages  to  the  adult  bones  may  be  seen  from  the  table 
given  on  page  187. 

With  regard  to  the  metacarpals  and  phalanges,  it  need  merely 
be  stated  that  each  develops  from  a  single  primary  center  for  the 
shaft  and  one  secondary  epiphysial  center.  The  primary  center 
appears  at  about  the  middle  of  the  shaft  except  in  the  terminal 
phalanges,  in  which  it  appears  at  the  distal  end  of  the  cartilage 
The  epiphyses  for  the  metacarpals  are  at  the  distal  ends  of  the  bones 
except  in  the  case  of  the  metacarpal  of  the  thumb,  which  resembles 

the  phalanges  in  having  its  epiphysis  at  the  pro.timal  end:     

.,^'^h  innominate  hone  appears  as  a  sdmtwhat  oval  .plate  of 

carfilige"  whose  loi^  axi»  is  "dtrecfed  abftosfat  "fight^anglw  to' the 


11 


i    ■41 
III 


i 


m 


i86 


DEVm.OnnWT   of  AWENBICniA*  SXELETON 


[I  I 


vertebral  column  and  which  is  in  close  relation  with  the  fourth  and 

fifth  sacral  vertebne.    As  development  proceeds  a  rotation  of  the 

cartilage,  accompanied  by  a  slight  shifting  of  position,  occurs,  so 

that  eventually  the  plate  has  its  long  axis  almost  parallel  with  the 

vertebral  column  and  is  in  relation  with  the  first  three  sacrals 

Ossification  appears  at  three  points  in  each  cartilage,  one  in  the 

upper  part  to  form  the  ilium  (Fig. 

IIS,  »0  and  two  in  the  lower  part, 

^he  anterior  of  these  givinp  rise  to 

the  pubis  {p),  while  the  posterior 

produces  the  ischium  (is).     At 

birth  these  three  bones  are  still 

separated  from  one  another  by  a 

Y-shaped  piece  of  cartilage  whose 

three  limbs  meet  at  the  bottom 

of  the  acetabulum,  but  later  a 

secondary  center  appears  in  this 

cartilage   and    unites    the   three 

bones  together.    The  central  part 

^  of  the  lower  half  of  each  original 

Fro.  US— TraOssmcATioMCENTsiis    cartilage  plate  does  not  undeiso 

..  t'c^'^^T"'^-     .      ^'"P'*^'*  chondrification,  but  re- 
«,»,«,  and  i,  Secondaiy  centen  for     _   .  ,  ' 

the  crest,  anterior  inferior  spine,  sym-    "la'DS  membranous,   constitutine 
E^&umt^'r^i,'"^^.)''''"'""'    "^  Obturator  membrane  which 

closes  the  obturator  foramen. 

In  addition  to  the  Y-shaped  secondary  center,  other  epiphysial 
centers  appear  in  the  prominent  portions  of  the  cartilage  such  as 
the  pubic  crest  (Fig.  iis,  «),  the  ischial  tuberosity  (rf),  the  anterior 
infenor  spine  (6)  and  the  crest  of  the  ilium  (o),  and  unite  with  the 
rest  of  the  bone  at  about  the  twentieth  year. 

The  ..'■m«r,  tibia,  andylJwfa  each  develop  from  a  single  primary 
center  for  the  shaft  and  an  upper  and  a  lower  epiphysial  center  the 
femur  possessing,  in  addition,  epiphysial  centers  for  the  greater 
and  lesser  trochanters  (Fig.  94).  The  patella  does  not  belong  to 
the  same  catsgoiy  as  the  other  bones,  but  resembles  the  pisiform 


DEVBLOPMBNT   OF  APPENDICtFLAB  SKELETON  187 

bone  of  the  carpus  in  being  a  sesamoid  bone,  developed  in  the  tendon 
of  the  quadriceps  extensor  cruris.  Its  cartilage  does  not  appear 
until  the  fourth  month  of  intrauterine  life,  when  most  of  the  primary 
centers  for  other  bones  have  abeady  appeared,  and  its  ossification 
does  not  begin  until  the  third  year  after  birth. 

The  tarsus,  like  the  carpus,  consists  of  a  proximal  row  of  thre- 
cartilages,  termed  the  Hbiale,  the  intermedium,  and  thefibulare,  and 
of  a  distal  row  of  four  larsalia.  Between  these  two  rows  a  single 
cartilage,  the  cenirale,  is  interposed.  Each  of  these  cartilages  ossifies 
from  a  single  centtr,  that  of  the  intennedium  early  fusing  with  the 
tibiale,  though  it  occasionaUy  remains  distinct  as  the  as  trigonum,  and 
from  a  comparison  with  lower  forms  it  seems  probable  that  the 
nbular  cartilage  of  the  distal  row  really  represents  two  separate 
elements,  there  being,  properly  speaking,  five  tarsalia  instead  ot 
four.  The  fibulare,  in  addition  to  its  primary  center,  possesses  also 
aa  epiphysial  center,  which  develops  at  the  point  of  insertion  of  the 
tendo  Achillis. 

A  comparison  of  the  carpal  and  tarsal  cartilages  and  their 
relations  to  the  aduh  bones  may  be  seen  from  the  following  table: 


ft 


Carpus 


Tarsus 


Cartilagea 


Bones 


Radiate 
Intermedium 

Navicular 
Lunate 

Uhuie 
Sesamoid  cartilage 

Triquetral 
Pisiform 

CentnJe 
Carpale     I 

Fuses  witll  navicular 
Gr.  multangular 

Carpale  II 
Carpale  III 
Carpale  IV  \ 
Catprie    v; 

Less,  multangular 
Capitate 

Bones 


Cartilages 


I 

j  Talus 

1  Calcaneus 

f  Tibiale 
I  Intermedium 
Fibulare 

Navicular 
.1st  Cuneiform 
3d  Cuneiform 
3d  Cuneiform 

Centrale 
Tarsale     I 
Tarsale   II 
Tareale  lU 

Cuboid 

Tarsale  IV 
Tarsale    V 

III 


111 


i88 


DEVELOPMENT   OF   THE   JOINTS 


The  development  of  the  metatarsals  and  phalanges  is  exactly 
sim^r  to  that  of  the  con-esponding  bones  of  the  hand  (see  p^jV 

nri^ril  "*"''°P°'«"*  «>*  «»•  Jolnts.-The  mesenchyme  which 
pnmanly  represents  each  vertebra,  or  the  skull,  or  the  skeleton  of 
a  imb,  .s  at  first  a  continuous  mass,  and  when  it  becomes  converted 
into  cartilage  th.s  also  may  be  continuous,  as  in  the  skull,  or  may 
appear  as  a  number  of  distinct  parts  united  by  unmodified  portions 
o  the  mesenchyme.  In  the  former  case  the  various  ossifications 
as  they  extend  w.U  come  into  contact  with  their  neighbors  and  will 

a'^«r?''  "  '''"  ""''"'^'^  ^''^  "•'"  '*''"">'•  f°™'°« 

When,  however,  a  portion  of  unmodified  mesenchyme  intervenes 
between  two  cartilages,  the  mode  6i  articulation  of  *he  bones  formed 
from  these  cartilages  will  vaiy.    The  intermediate  mesenchyme 
may  m  time  undergo  chondrification  and  unite  the  bones  in  an 
almost  immovable  articulation  known  as  a  synchondrosis  (e.  g    the 
articulation  of  the  fi«t  rib  with  the  sternum);  or  a  cavity  may  appear 
m  the  center  of  the  intervening  cartilage  so  that  a  slight  amount  of 
movement  of  the  two  bones  is  possible,  forming  an  amphiarthrosis 
(«.  g.,  the  symphysis  pubis);  or,  finally,  the  intermediate  mesen- 
chymej^y  not  chondrify,  but  its  peripheral  portions  may  become 
converted  mto  a  dense  sheath  of  connective  tissue  (Fig    ii6    c) 
which  surrounds  the  adjacent  ends  of  the  two  bones  like  a  sleeve 
fonnmg  the  articular  capsule,  while  the  central  portions  degenerate 
to  form  a  cavity.    The  bones  which  enter  into  such  an  articulation 
are  more  or  less  freely  movable  upon  one  another  and  the  joint  is 
termed  a  dtarthrosis  (e.  g.,  the  knee-  or  shoulder-joint) 

In  a  diarthrosis  the  connective-tissue  cells  near  the  imier  surface 
of  the  capsule  arrange  themselves  in  a  layer  to  form  a  synovial 
membrane  for  the  joint,  and  portions  of  the  capsule  may  thicken 
to  form  sptual  bands,  the  reinforcing  ligaments,  while  other  strong 
fibrous  bands,  which  may  pass  from  one  bone  to  the  other,  forming 
accessory  ligaments,  are  shown  by  comparative  studies  to  be  in  many 
casesdegenerated  portions  of  what  were  originally  muscles.  , 

In  certain  diarthroses.  such  as  the  temporo-mandibular'and 


DEVELOPMENT  OF   THE  JOINTS  igg 

Th^^  J^  u^"'  '^°''  ""'  <l'^«^n'>--'e.  but  it  is  converted  into  a 
fibro^art.Iage,  between  each  surface  of  which  and  the  adjacent 
bone  there  .s  a  cavty.  These  interarticular  cartilages  seem,  n  the 
.en,o<lav,cular  joints,  to  represent  the  sternal  ends  of  I  bone 

rrl'°^'°Krr-r'?'''''''  '^"^  '"°^"  ^^  '^e  precoracM,  but  it 
seems  doubtful  ,f  those  of  the  temporo-mandlbular  and  knee 


i-  .iiu  J7-,  csTtuages  of  the  first  and  second  phalanges.— (JVfcAo/aj.) 

joints  have  a  similar  significance,  the  most  recent  observations  on 
m^nchym?"*'"*  *""^''°*  '°  ^"''"'  *""  ^'""^  *""=  intermediate 

di^SLi^f'^  ^'  °'  <Jevdopment  it  is  evident  that  the  cavities  of 
toSte  forte  "'^r^^^^^y  ^°^'='»  '""^  *^^"  "''l'^.  except  whe?e 
Svfdes  ofcS  ".^"'^''"^..^Wch,  at  first  sight,  appear  to  travel  the 
bv  a^^Ih  nf       ^  ",'''  ?."  '"  "^'y  "eluded  from  them,  being  lined 

traverse  l^^^l"^"^  °  "^^  '""8  ''^'d  "^  *«  biceps,  wWch  s^ms  to 
caS  ^™n  "vt'^r"'  "'  *"  **"=  f«"^  ^'"''''ly  ou'^de  the  articular 
DusS  th^frH  i*  "  ''"f-  v^"'"  "  ''"''^  '"  'o^^d  the  joint  cavity' 
hT^cl^sule  whth  lT"''^'°"  ''-  ^  "*"' ''  "^^  ''t  fi'^'  i"  »  8'oove 
MParaterfrnm  ?fc  "/"  °,°  5^°™'  converted  into  a  canal  and,  finaUy, 
separates  from  the  rest  of  the  capsule  except  at  its  two  extremitii 


'  i. 
'  i 


190 


tlTEKATUIE 


forming  a  cylindrical  canal,  open  at  either  end,  traversing  the  joint  cavit/ 
and  containing  the  tendon  of  the  biceps. 

The  ligamentum  teres  of  the  hip-joint  is  similarly  excluded  from  iae 
joint  cavity  by  a  sheath  of  synovium,  which  extends  outward  around  it 
from  the  bottom  of  the  acetabular  fossa  to  the  depression  in  the  head  of 
the  femur,  and  in  the  knee-joint  the  crucial  ligaments  are  also  excluded 
from  the  cavity  by  a  reflection  of  the  synovium.  This  joint,  indeed,  is 
m  flie  fetus  incompletely  divided  into  two  parts,  one  corresponding  to 
each  femoral  condyle,  by  a  partition  which  extends  backward  from  the 
patellar  ligament  to  the  crucial  ligaments,  remains  of  this  partition 
persisting  in  the  adult  as  the  so-called  ligamentum  mucosum. 


if 


LITERATURE. 

I 

C.  R.  Bahdein:  "The  Development  of  the  ThonKicVCTtebnc  in  Man," /I »i(r./o«n.. 

Anai.,  IV,  1905. 
C.  R.  Bakdeen:  "Studies  of  the    Development  of  the  Human  Slceleton,"   Amtr 

Joum.  Anal.  IV,  igos. 
C.  R.  Bardeen:  "Early  Devebpment  of  the  Cervical  Vertebra:  and  the  Base  of  the 

Occipital  Bone  in  Man,"  Avur.  Joum.  Anal.,  vni,  1908. 
C.  R.  Ba«deen:  "Vertebral  Regional  Determination  in  Young  Human  Embryos," 

Anal.  Rtcord,  n,  1908. 
E.  T.  BElt.:  "On  the  Histogenesis  of  the  Adipose  T.i5ue  of  the  Oi,"  Amtr.  Joum. 

Anal.,  IX,  1909. 
A.  BZEHAYS:  "Die   Entwicklungsgeschichte   des    Kniegelenks   de»   Menschen    mit 

Bemerkungen  Uber  die  Gdenke  im  AUgemeinen,"  Marpkolog.  Jahrhuck,  rv,  1878. 
E.  Duasy:  "Zur  EntwicHungsgeschichte  des  Kopfcs  des  Menschen  und  der  hfiheren 

Wirbelthiere,"  Tubingen,  1869. 
...  Fawcett:  "On  the  Development,  Ossification  and  Grorth  of  the  Pahlte  Bone," 

Joum.  Anal,  and  Pkys.,  xi,  1906. 
E.  Fawcett:  "Notes  on  the  Development  of  the  Human  Sphenoid,"  Joum.  Anal. 

and  Pkys.,  XLiv,  1910. 
E.  Fawcett:  "The  Development  of  the  Human  Maxilla,  Vomer  and  Paiaseptal  C 

tilages,"  Joum.  Anal,  and  Pkys.,  xiv,  igii. 
A.  FtoUEP:  "Zur  Entwicklungsgeschichte  der  Wirbelsftule,  insbcsondere  des  Atks 

und  Epistropheus  und  der  Occipitabegion,"  Ankrvjar  Anal,  und  Pkysial.,  Anal. 

AUk.,  1886. 
E.  Gao»p:  "Alte  Probleme  und  neuere  Arbeiten  (iber  den  Wirbeltierschsdel,"  Ergtb. 

da  Anal,  und  Enlwicklungsgesck.,  x,  1901. 
C.  GEOEHBAtw:     Ein  Fall  von  erblichem  Mangel  der  Pars  acromialis  Clavicute,  mit 

Bemerkungen  Uber  die  Entwicklung  der  Clavicula,"  Jmaisckt  Zeilsckr. /ilr  medic. 

Wissmsck,,  I,  1864. 
J.  GoLOwnraKl:  "Zur  Kenntnis  der  Histogenese  der  BindcgewebsfibriUcn,"  Anal. 

HtftCt  xxxm,  1907. 


UTEIATUU 


191 


"^T,!^"  T"^"-  "f"""™'"«'<"=EntwickrlungderEx,remi«,end«M™«hen 

"  "rrr^ri™'  -  r  °--«-' »'  *«  H„_  u..„  ,.„, 
"  "d^";;,.?:;^'^;::"  jr:  i-j-™-  "■"''■^"'  ■--'  '^'-" «-  »•"'•'«' 

"■  "x"™;  ;^;'"^'"'""^  ">-'  "«  •^"•-''lung  des  Bindegew^bes,"  An^.  He/,.. 
A.  M.  Paiekson:  ■■  The  Human  Stemum,"  Liverpool,  ,904 

Rambadt  et  Renault:  "Origine  «  divdoppem^t  d«  Os,"  Paris.  i86a 

Etem«te  ,m  Menscbl,ch,-„  Carpu.  (and  Tarsus)."  M^/Mog.  ArMUn,  v.  ,S^. 


m 


T93 


LITIKATITKE 


K.  ToiDT  J«.:  "EntwicUung  tmd  Stniktur  des  mouchUchai  JochbOatt," SilMttOer: 

k.  Acad.  Wiuauch.  Wim,  Matk.-iulurmss  Kl,  cxi,  ipoj. 
A.  VraooJADOFF:  "D^veloppcment  de  rarticuUtion  tempoio-maxilUire  cha  rhomme 

dans  la  (Xriode  intrauterine,"  InUmal.  Umalssclir.  Anal.  Phys.,  xxvn,  1910. 
R.  H.  Whitihkad  and  J.  A.  Waddell:  "  The  Early  Development  of  the  MamnulUn 

Sternum,"  Amer.  Joum.  Anat.,  xil,  1911. 
L.  W.  WaUAMS:  "The  Later  Development  of  the  Notochonl,"  Amer.  Jam  Anal 

viii,  1908.  ' 

E.  Zuckirkandl:  "Ueber  den  Jacobaonschen  Knorpel  und  die  Ouifikation  des 

Pflugscharbeine5,"5i<«6.  Akad.  Whs.  Wien.,  cxvii,  1908. 


CHAPTER  VIII 
THE  DBVEL0PME5X  OF  THE  MUSCULAR  SYSTEM 

conbol  of  the  central  nervous  system  atdTh  ""''  "  ""''*'  "'«' 

controUed  by  the  sympathetL  C";:"!:^!"!     ^ ^  "'^ 

skin,  in  the  walls  of  the  digestive  tZ-f  TZ  '*  '°"'"'  '"  "»« 
Phatics,  and  in  connection  Shth!^'  •?'  '"^"^^^^^'^  ''"d  lyn>. 
the  walls  of  the  heart  ^n^^e  nlfr  """""^  ''PP*™'"^-  I" 
«garded  as  a  third  for^cWtri^*^:^  •"'"';'  ^"''"'^""^ 
the  sympathetic  system  a^d  yett^ttriSed?irh'"  ~"'~'  °^ 
origin  much  more  nearly  alhed  ^  *!.  '        '  ^•"^^ver,  in  its 

striatedformoftissue,lnLmSe  nJllrT""'"'!*'"'  """  '°  *«^ 

the  exception  of  AesphincterTndSor^S'*'^^'""'--^''^ 
of  the  sudoriparous  glands  S  t^f  ?T' ""'^  *^ '"»'^'«=« 
aU  the  non-stTted  J^tC:'.,Z  Syt  trr^™' 
version  of  mesenchyme  cells  into  musTfil^  '  rt  V''' ""' 
this  process  have  been  worked  out  by  McGHI  ^;  .k  ''''*''*  °^ 
of  the  digestive  and  respiratory  tracts  of  tLn  ^  '  «"^^«lature 
The  mesenchyme  surroundin7thrl  ^r"''  "'  ''^  ^°"°''«= 

a  loose  syncytia  ^^fZ  ^n^  T  '"  '^'''  '"""^'^  '^  ''^  ^^t 
tissue  is  to  form  a  Sen^t^i  oHh°  'T"'  """^  "^^  ™"-'« 
by  an  elongation  of  ^^t  "n  hy^  "^^^  "^T-^""'"^^'^ 
the  muscle  layers  become  clearly  dktin  '  km  r  "'"''^''  '"  *'" 
ing  undifferentiated  tissue1^r«  T«f  F  ,^^7 '^^  "^'^'"^^^ 

a"a~.n„mbe=iXpflT;;-:otr.l^ 


194         HYSTOOCNESIS   OF   NON-STSUTED   HUSCUIAR  TISSUE 


FlO.  117. — LONGITUDINAl.  SECTION  OF  THE  LOWEX  PAST  OF  THE  OESOPHAGUS  OF  \ 
PlO  ElIBEYO  OF  15  HU,  SHOWING  THE  HISTOGENESIS  OF  THE  NOK-STMATEU 
MUSCULATUEE. 

6,  Basement  membrane;  t,  epithelium;/c-,  coane  fibril;//.,  fine  fibril;  fa,  ganglion 
ot  Auerbach's  plems;  gm,  ganglion  of  Meissner's  pleius;  m,  mesenchyne;  mti:, 
muKularis  mucosa;  pb,  protoplasmic  bridge;  vf,  varicTise  fibril.— (ifeCtU.) 


throughout  the  cytoplasm  TthlT'  .  7  ""  """''""^  ""''"""'X 
as  development  pr^T  wi  ^f '  ""k*"?  '"^'«'*^ '"  '""">«'• 
be  entixely  wanting  i^Z  Id^h  Itsu?'"  '"""  •^™™"'  '^^'^  "«y 

undS:^he  dLS'oSt  r  'L"^"  '""^''  '••-'  ^■"•'  'o 
interstifal  connect  tTlf  "'^"^'^  ^"'^  """"P'y  ">  form  the 
which  usuaUy  divides  the  mus- 
cle cells  into  more  or  less  dis- 
tinct bundles.  Traces  of  the 
original  syncytial  nature  of 
the  tissue  are  to  be  seen  in 
the  interceUular  bridges  that 
occur  between  the  non-striated 
muscle  cells  of  many  adult 
forms. 

The  cells  from  which  the 

heart    musculature    develops 

are  at  first  of  the  usual  well 

defined  embryonic  type,  but, 

as  development  proceeds,  they 

become  irregulariy  stellate  in 

form,  theprocessesof  neighbor- 

'ng  cells  fuse  and,  eventually, 

there  is  formed  a  continuous 

-  th.7  m  lemS  .i  t  m  .         .1";  »*«"l  C.I1.,  o,  „y.u^ 


isilsF---™: 


196         RUT00ENEU8   or   N0M-8TUATU>  UVKXTLAti  IIUUX 

through  considerable  stretches  of  the  syncytium,  without  regard  to 
the  original  cell  areas. 

The  fibrils  multiply,  apparently  by  longitudinal  division,  and 
arrange  themselves  in  circles  around  areas  of  the  syncytium  (com- 
pare Fig.  119).  As  the  muhiplication  of  the  fibrils  continues  those 
newly  formed  arrange  themselves  around  the  interior  of  each  of  the 
original  circles  and  gradually  occupy  the  entire  cytoplasm,  or  saico- 
plasm  as  it  may  now  be  termed,  except  immediately  around  the  nuclei 
where,  even  in  the  adult,  a  certain  amount  of  undifferentiated  sarco- 
plasm  persists.    The  fibrils  when  first  formed  are  apparently  homo- 


^^^f|^' 


Fig.  119. — Caoss-sccnoN  of  a  Muscle  »oh  the  Thigh  or  a  Fig  EHstyo  75  uu. 

Long. 

A,  Central  nucletu;  5,  new  peripheral  nucleus. — (Siacallum.) 


geneous,  but  later  they  become  differentiated  into  two  distinct  sub- 
stances which  alternate  with  one  another  throughout  the  length 
of  the  fibril  and  produce  the  characteristic  transverse  striation  of  the 
tissue.  Finally  stronger  interrupted  transverse  bands  of  so-called 
cement  substance  appear,  dividing  the  tissue  into  areas  which  havi; 
usually  been  regarded  as  representing  the  original  myoblasts,  but 
are  really  devoid  of  significance  as  cells,  the  tissue  remaining, 
strictly  speaking,  a  syncytium. 


HI8T00.NM,,  O,   8TMAT.D   MtTSCU!   TISg«,E  ,5, 

Th«  HiitogniMii  of  striated  MiucU  TlMu,._The  hi.jo 
hat  which  ha.  just  been  described  for  the  heart  mu^Ie      rhe«Ta 

!m.„^.  v.  t^"'*  •'^  'onptudinal  division  and  a  primary 
Ta^S'  If  fibriU  a«.„nd  the  periphery  of  areas  of  s^^^ 
puarn  (Fig.  19),  each  of  which  represents  a  muscle  fiber     In 

^^Z  7  ''  ""  '""*  P"""'™"""  °f  'he  nuclei  of  the  originl" 
myoblasts,  the  new  nuclei  arranging  themselves  more  o   less  1^ 

ment  Kw  ,      "i  ""'*' '"  "••=  """'  '""'^'^>  'he  develop- 

TZ\  "J""her  "mphcated  by  a  process  of  degeneration  which 

^^«tS°bTsl7"?H''*"'  "  ""'  '"'"'^'-  °^  -"»■"  fihe«  - 
tofreS  th?„,i,  .  ^'k"""^  "'"'"^  '■"  *hich  the  fibrils  have 
IXnd  hni  have  become  pale  and  the  sarcoplasm  vacuo- 

toted  and  homogeneous.  Uter  the  degenerated  tissue  seems  to 
2;rw'r:;"""'^  ■^"^  mesenchymatous  connective  Ze  pows  n 

So  long  as  the  formation  of  new  fibrils  continues,  the  increase  in 

nC^^^The"!';  T'^  1'  '•"'''"^  •'"^  '°  "^  -«^-  ^«-"o  an 
ionTh^  »I.  r  °"""^'  °^  ^'^"'  ^hich  results  from  the  divi- 
^n  of  those  already  existing.  Subsequently,  however,  this  mode  of 
growth  ceases,  the  further  increase  of  the  muscle  depend  „guw„  an 
.ncrease  m  sue  of  its  constituent  elements  (Macallum) 

The  Development  of  the  Skeletal  Muscles.-It  has  alreadv 
been  pomted  out  that  all  the  skeletal  muscles  of  the  bodv  wiJ.  S 
e«ept.on  of  those  connected  with  the  branchial^^l^ardlS 
from  the  myotomes  of  the  mesodermic  somites,  ev;n  theTmb 

."reTrom^f  hS^'"*  T  ''"  °"«'"'  ^'">-«h'  the  celf  o  The 
s  ue  from  which  the  muscles  of  the  limb  buds  form  lack  an  epithe- 
l.al  anangement  and  are  indistinguishable  from  the  somaUc  Zen 
chyme  wh.ch  forms  the  axial  cores  of  the  limbs 

The  various  fibers  of  each  myotome  are  at  first  loosely  arranged, 


198 


DEVELOniXNT  or  SKZLKTAL  MDICUf 


but  later  they  become  more  compact  and  are  arranged  parallel  with 
one  another,  their  long  axes  being  directed  antero-posteriorly. 
Thii  stage  i°  ^Iso  transitory,  however,  the  fibers  of  each  myotome 
iwdergoing  \ai  <us  modifications  to  produce  the  conditions  existing 
in  the  adult,  in  which  the  original  segmental  arrangement  of  the 
fibers  can  be  perceived  in  comparatively  few  muscles.  The  exact 
nature  of  these  modifications  is  almost  unknown  from  direct  obser- 
vation, but  since  the  relation  between  a  nerve  and  the  myotome 
belonging  to  the  same  segment  is  established  at  a  very  early  period 
of  development  and  persists  throughout  life,  no  matter  what  changes 
of  fus'sn,  splitting,  or  migration  the  myotome  may  undergo,  it  is 
possible  to  tra:e  out  more  or  less  completely  the  history  of  the  various 
myotomes  by  determining  their  bcgmetital  inner  .'ation.  It  is  known, 
for  example,  that  the  latissimus  dorsi  arises  from  the  seventh  and 
eighth'*'  cervical  myotomes,  but  later  urdergoes  a  migration,  becom- 
ing attached  to  the  lower  thoracic  and  lumbar  vertebrc  and  to  the 
crest  of  the  ilium,  far  away  from  its  place  of  origin  (Mall),  and  yet 
it  retains  its  nerve-supply  from  the  seventh  and  eighth  cervical 
nerves  with  which  it  was  originally  associated,  its  nerve-supply 
consequently  indicating  the  extent  of  its  migration. 

By  following  the  indications  thus  afforded,  it  may  be  seen  that 
the  changes  which  occur  in  tlie  myotomes  may  be  referred  to  one  or 
more  of  the  following  processes: 

1.  A  longitudinal  splitting  into  two  or  more  portions,  a  process 
well  illustrated  by  the  trapezius  and  stemomastold,  which  have 
differentiated  by  the  longitudinal  splitting  of  a  single  sheet  and 
contain  therefore  portions  of  the  same  myotomes.  The  sterno- 
hyoid and  omohyoid  have  also  differentiated  by  the  same  process, 
and,  indeed,  it  is  of  frequent  occurrence. 

2.  A  tangential  splitting  into  two  or  more  layers.  Examples  of 
this  are  also  abundant  and  are  afforded  by  the  muscles  of  the  fourth, 
fifth,  and  sixth  layers  of  the  back,  as  recognized  in  English  text-books 

*  This  enumezation  U  baaed  on  csnveniencc  in  associating  tlie  myotomes  with  th< 
'lervea  wliich  supply  them.  The  myotomes  mentioned  are  those  which  correspond  ti  > 
the  sixth  and  seventh  cervical  vertebise. 


of  the  thorax  ^       in««o.t.l  muscle,  and  the  tran.venus 

by  the  r^ni/aEis    which     T""l"'!'  '"'*  "'"  '■"«»'«'«« 
ventr.1  portions  StheZ/  "  '°'''""*  ^^  '^'  f""'<»>  <>'  the 

the  «'r^JZ^^:jz:-:jt-''  -^"'""-^  -  ^^ 

whose  hisLryls  ah^Sr^* ',:,^7''  "•''.»'*  '»««™"^  dors!, 
shown  by  f  "  serratr!nf,w  f^  '"'  ""*  "  "  »'»*'  beautifully 
extendedV^^J^thet^^'f ^^^  '"^""'' '""' »'  *''-'•  "ave 
derived.  '"  °'  '^'^  **«■"«'»  '«>•»  which  they  are 

ThifplSrtje^'ar'""  ".  ''•'  ^""'^  °'  "^  "^"'-e- 
of  the  «u"ui^sSem  in  T  'Tu"^''''  P*"  '"  '^'  -"'""o" 
■nally  deg«e"ates  ?  wl        ^'"'•'"""-    When  a  muscle  nor- 

"rsj:j:r;i=-fro?j' '-  ^^^^^^^ 

is  formedln  thirZ^?.!  '''  T^  °'  "'^  "ccipito-frontalis 
monkeys,  and  a  S^"  ,  '"T"*'^" '»  ™eh  forms  as  the  lower 

Which  ^.uptrsritrtxirarr--^ 

one  ^f  S'l  rE^oftrt     ''r  "T^"  ^'■"'  "^  "-^^  "^ 
nective  tissue  compared  w  th Th  ''  "'  «''''"  ^•°°""'  "'  """ 

fonncr  is  ve^  sS  T     ,    ""T  °'  "■"'*^"'^'  t'^^---'  «  the 
.issue  strucJJesSatfe  fl"^'^""  ,^'»«  'hat  these  com^ective- 

of  the  muscurtiss^^r/tri         r'  'fr'"'^^  ^^P^'^^"'  P»«'<">« 
accessory  linemsr/^  ^'' ''•™  (Bardeleben).    Many  of , he 

apparently  fwTtheir  nn^      ?""'''""' '''"'  diarthrodial  joints 
PParentiy  owe  their  ongin  to  a  degeneration  of  muscle  tissue,  the 


Ml 


*°°  IHE   muNK   MUSCULATlniE 

fibular  lateral  ligament  of  the  knee-joint,  for  instance,  being  probably 
a  degenerated  portion  of  the  peroneus  longus,  while  the  sacro- 
tuberous  ligament  appears  to  stand  in  a  similar  relation  to  the  long 
head  of  the  biceps  femoris  (Sutton). 

6.  Finally,  there  may  be  associated  with  any  cf  the  first  four 
processes  a  change  in  the  direction  of  the  muscle-fibers.  The 
onginal  antero-posterior  direction  of  the  fibers  is  retained  in  com- 
paratively few  of  the  adult  muscles  and  excellent  examples  of  the 
process  here  referred  to  are  to  be  found  in  the  intercostal  muscles 
and  the  muscles  of  the  abdominal  walls.  In  the  musculature 
associated  with  the  branchial  arches  the  alteration  in  the  direction 
of  the  fibers  occurs  even  in  the  fishes,  in  which  the  original  direction 
of  the  muscle-fibers  is  very  perfectly  retained  in  other  myotomes,  the 
branchial  muscles,  however,  being  arranged  parallel  with  the 
branchial  cartilages  or  even  passing  dorso-ventraUy  between  the 
upper  and  lower  portions  of  an  arch,  and  so  forming  what  may  be 
regarded  as  a  constrictor  of  the  arch.  This  alteration  of  direction 
dates  back  so  far  that  the  constrictor  arrangement  may  weU  be 
taken  as  ihe  primary  condition  in  studying  the  changes  which  the 
branchial  musculature  has  undergone  in  the  mammalia. 

It  would  occupy  too  much  space  in  a  work  of  this  kind  to  con- 
sider in  detail  the  history  of  each  one  of  the  skeletal  muscles  of  the 
human  body,  but  a  statement  of  the  general  plan  of  their  develop- 
ment will  not  be  out  of  place.  For  convenience  the  entire  system 
may  be  divided  into  three  portions— the  cranial,  trunk  and  limb 
musculature;  and  of  these,  the  trunk  musculature  may  first  be 
considered. 

The  Trunk  Musculature.-It  has  already  been  seen  (p.  82) 
that  the  myotomes  at  first  occupy  a  dorsal  position,  becoming 
prolonged  ventrally  a„  development  proceeds  so  as  to  overlap  the 
somatic  mesoderm,  until  those  of  opposite  sides  come  into  contact 
m  the  mid-ventral  line.  Before  this  is  accomplished,  however,  a 
longitudinal  splitting  of  each  myotome  occurs,  whereby  there  is 
separated  off  a  dorsal  portion  which  gives  rise  to  a  segment  of  the 
dorsal  musculature  of  the  trunk  and  is  supplied  by  the  ramus  dorsali- 


™k  trunk  MUSCUIATDBE 


portion  is  ™uch  jl  dt  nTLTC^U  th'"  > '°""  ""'  ^'="''''' 
portions  being  separated  hv  V  K  ^  "  "  "  '"  '""»'  ""e  two 

extending  the'eX  C/o  th™  k  /nd'b  °'  '^~"'^^  '■■^^"«' 
inner  edge  to  the  transverse  proLrof  tt  '''"V"'"'^  ""'  ''^ 
«.  i.  beco.es  continuor:^%rc:C^^;r:- 


dermis  along  a  line  known  as  the  lateral  !,„,      t 

portion  is  also  much  sm^iut  •  ^"  ™*°  "•«  «*»"«' 


i^'i 


IHZ   TUTOiZ  MUSCULATUItE 


composed  of  muscles  belonging  to  the  limb  system.  Further 
longitudinal  and  tangential  divisions  and  a  fusion  of  successive 
myotomes  bring  about  the  conditions  which  obtain  in  the  adult 
doisal  musctilature. 

While  the  myotomes  are  still  some  distance  from  the  mid-ventral 
line  another  longitudinal  division  affects  their  ventral  edges  (Fig. 
lao),  portions  being  thus  separated  which  later  fuse  more  or  less 
perfectly  to  form  longitudinal  bands  of  muscle,  t'^ose  of  opposite 
sides  being  brought  into  apposition  along  the  mid-ventral  line  by 
the  continued  growth  ventrally  of  the  myotomes.  In  this  way  are 
formed  the  rectus  and  pyramidalis  muscles  of  the  abdomen  and  the 
depressors  of  the  hyoid  bone,  the  gfcnio-hyoid  and  genio-glossus* 
in  the  neck  region.  In  the  thoracic  region  this  rectus  set  of  muscles, 
as  it  may  be  termed,  is  not  represented  except  as  an  anomaly,  its 
absence  being  probably  correlated  with  the  development  of  the 
sternum  in  this  r^on. 

The  lateral  portions  of  the  myotomes  which  intervene  between 
the  dorsal  and  rectus'  muscles  divide  tangentially,  producing  from 
their  dorsal  portions  in  the  cervical  and  lumbar  regions  muscles, 
such  as  the  longus  capitis  and  colli  and  the  psoas,  which  lie  beneath 
the  vertebral  column  and  hence  have  been  termed  hyposkeletal 
muscles  (Huxley).  More  ventrally  three  sheets  of  muscles,  lying 
one  above  the  other,  are  formed,  the  fibers  of  each  sheet  being 
arranged  in  a  definite  direction  differing  from  that  found  in  the  other 
sheets.  In  the  abdomen  there  are  thus  formed  the  two  oblique  and 
the  transverse  muscles,  in  the  thorax  the  intercostals  and  the  trans- 
versup  thoracis,  while  in  the  neck  these  portions  of  some  of  the  myo- 
tomes disappear,  those  of  the  remainder  giving  rise  to  the  scaleni 
muscles,  portions  of  the  trapezius  and  stemomastoid  (Bolk),  and 
possibly  the  hyoglossus  and  styloglossus.  In  the  abdominal  region, 
and  to  a  considerable  extent  in  the  neck  also,  the  various  portions  of 
myotomes  fuse  together,  but  in  the  thorax  they  retain  in  the  inter- 
costab  their  origiral  distinctness,  being  separated  by  the  ribs. 

•  This  muscle  is  supplied  by  the  hjpogloss*!  nerve,  but  for  the  present  purpose  it  is 
convenient  to  regard  this  as  a  spinal  nerve,  as  indeed  it  primarily  is. 


THK  TXDNX  MUSCULATUKE 


ao3 


lit 


304 


THE   HUNK  IIDSCCIATUSE 


The  table  on  page  203  will  show  the  relation  of  the  various  irunk 
muscles  to  the  portions  of  the  myotomes. 

The  intimate  association  between  the  pelvic  girdle  and  the  axial 
skeleton  brings  about  extensive  modifications  of  the  posterior  trunk 
myotomes.  So  far  as  their  dorsal  portions  are  concerned  probably 
all  these  myotomes  as  far  back  as  the  fifth  sacral  are  represented  in 
the  sacro-spinalis,  but  the  ventral  portions  from  the  first  lumbar 
myotome  onward  are  greatly  modified.  The  last  myotome  taking 
part  in  the  formation  of  the  rectus  abdominis  is  the  twelfth  thoracic 
and  the  last  to  be  represented   in  the  lateral  musculature  of  the 


wfj,"^^""'"''  "^°'°''  '"'  ExsKVos  or  (A)  Two  Mo.nths  and  (B)  Fouk  to 
Five  Months,  showing  the  Development  of  the  Ferine/     Moscura 
<fe,  Nervus  dorsalis  clitoridis;  #  pudendal  nerve;  m,  sphincter  ani',  sc  sphincter  doaoe- 
sv,  sphincter  vagtns. — (Popowsky.)  ' 

abdomen  is  the  first  lumbar,  the  ventral  portions  of  the  remaining 
lumbar  and  of  the  first  and  second  sacral  myotomes  either  having 
disappeared  or  being  devoted  to  the  formation  of  the  musculature 
of  the  lower  limb. 

The  ventral  portions  of  the  thirti  and  fourth  sacral  myotomes  are 
represented,  however,  by  the  levator  ani  and  cocvygeus,  and  are  the 
last  myotomes  which  persist  as  muscles  in  the  humin  body,  although 
traces  of  still  more  posterior  myotomes  are  to  be  found  in  muscles 
such  as  the  curvator  coccygis  sometimes  developed  in  connection 
with  the  coccygeal  vertebras. 

The  perineal  muscles  and  the  external  sphincter  ani  are  also 


IHE  CBANlAl  MCSCDIATCBE 


265 


developments  of  the  third  and  fourth  (and  second)  sacral  myotomes. 

m  .^  T-  T  t  """"  ^"^  P-  '^'>  '■"  ""'■"  P'<^^»'.  a  sheet  of 
muscles  lymg  dose  beneath  the  integument  forms  a  sphincter  around 
.U  opemng  q^,g.  „i).  On  the  development  of  the  partition  which 
?, Tl!^'.   '.  r"  """  '"''*'  """^  urinogenital  portions,  the  sphincter 

n£t  r  '  I, ,  T'  ^'"'"'"  P°"'°°  P*'^''^'i''«  «  'he  e:ctemal 
sphmcter  am,  wh.le  the  anterior  part  graduaUy  differentiates  into  the 
vanous  penncal  muscles  (Popowsky). 

rl,,?'  ^l"^"-  """"I't^W-As  was  pointed  out  in  an  earlier 
cuap.c,  the  existence  of  distinct  mesodermic  somites  has  not  yet 
been  completely  demonsU-ated  in  the  head  of  the  human  embryo, 
bu  m  lower  forms,  such  as  the  elasmobranch  fishes,  they  are  clearlv 
d.stmgu.shable  and  .t  may  be  supposed  that  their  indistinctness  in 
man  ,s  a  secondary  condition.    Exactly  how  many  of  these  somites 
are  represented  m  the  mamm-Jian  head  it  is  impossible  to  say,  but 
It  seems  probable,  from  comparison  with  lower  forms,  that  there  is 
a  considerable  number.    The  majority  of  them,  however,  early 
undergo  degeneration,  and  in  the  adult  condition  only  three  are 
wcogmzable,  two  of  which  are  pneoral  in  position  and  one  postoral. 
The  myotomes  of  the  anterior  pneoral  segment  give  rise  to  the 
muscles  of  the  eye  supplied  by  the  third  cranial  nerve,  those  of  the 
posterior  one  furnish  the  superior  oblique  muscles  innervated  by  the 
fourth  nerve,  while  from  the  postoral  myotomes  the  lateral  recti 

'?^  K  u^*'  "'''*  °"^''  "''  developed.  The  muscles  sup- 
plied by  the  hypoglossal  nerve  are  also  derived  from  myotomes  but 
they  have  already  been  considered  in  connection  with  the  trunk 
musculature. 

The  remaining  muscles  of  the  head  differ  fron  aU  other  voluntary 
muscles  of  the  body  in  the  fact  that  they  a.e  derived  from  the 
branchiomeres  formed  by  the  segmentation  of  the  cephalic  ventral 
mesoderm.  These  muscles,  therefore,  are  not  to  be  regarded  as 
equivalent  to  the  myotomic  muscles  if  their  embryological  origin  is 
to  be  teken  as  a  criterion  of  equivalency,  and  in  their  case  it  wouM 
seem,  from  the  fact  that  they  are  innervated  by  nerves  fundamentaUy 
distinct  from  those  which  supply  the  myotomic  muscles,  that  this 


.106 


IHZ  CSANIAL  MDSCCLATUSX 


criterion  is  a  good  one.  They  must  be  regarded,  therefore,  as 
belonging  to  a  special  category,  and  may  be  termed  brauckiomeric 
muscles  to  distinguish  them  from  the  myotomic  set. 

If  their  embiyological  origin  be  taken  as  a  basis  for  homology,  it  is 
clear  that  they  should  be  regarded  as  equivalent  to  the  muscles  derived 
from  the  ventral  mesoderm  of  the  trunk,  and  these,  as  has  been  seen, 
are  the  non-striated  muscles  associated  with  the  viscera,  among  which 
may  be  included  the  striated  heart  muscle.  At  first  sight  this  homology 
seems  decidedly  strained,  chiefly  because  long-continued  custom  has 
regarded  the  histological  and  physiological  peculiarities  of  striated 
non-striated  muscle  tissue  as  fundamental.  It  may  be  pointed  »..»., 
however,  that  the  branchiomeric  muscles  are,  strictly  speaking,  visceral 
muscles,  and  indeed  give  rise  to  muscle  sheets  (the  constrictors  of  the 
pharynx)  which  surround  the  upper  or' pharyngeal  portion  of  the  digestive 
tract  It  is  possible,  then,  that  the  homology  is  not  so  strained  as  niight 
appear,  but  further  discussion  of  it  may  profitably  be  deferred  until  the 
cranial  nerves  are  under  consideration. 

The  skeleton  of  the  first  branchial  arch  becomes  converted  partly 
into  the  jaw  apparatus  and  partly  into  auditory  ossicles,  and  the 
muscles  derived  from  the  corresponding  branchiomere  become 
the  muscles  of  mastication  (the  temporal,  masseter,  and  pterygoids), 
the  mylohyoid,  anterior  belly  of  the  digastric,  the  tensor  veli  palatini 
and  the  tensor  tympani.  The  nerve  which  corresponds  to  the  first 
branchial  arch  is  the  trigeminus  or  fifth,  and  consequently  these 
various  muscles  are  supplied  by  it. 

The  second  arch  has  corresponding  to  it  the  seventh  nerve,  and 
its  musculature  is  partly  represented  by  the  stylohyoid  and  posterior 
belly  of  the  digastric  and  by  the  stapedius  muscle  of  the  middle  ear. 
From  the  more  superficial  portions  of  the  branchiomere,  however,  a 
sheet  of  tissue  arises  which  gradually  extends  upward  and  downward 
to  form  a  thin  covering  for  the  entire  head  and  neck,  its  lower  portion 
giving  rise  to  the  platysma  and  the  nuchal  fascia  which  extends 
backward  from  the  dorsal  border  of  this  muscle,  while  its  upper  parts 
become  fhe  occipito-frontalis  and  the  superficial  muscles  of  the  face 
(the  muscles  of  expression),  together  with  the  fascise  which  unite 
the  various  muscles  of  this  group.  The  extension  of  the 
platysma  sheet  of  muscles  over  the  face  is  well  shown  by  the 


IHE  CRANML  MDSCDLATinffi 


307 


teF"° -= "—  ->^°^™T«?'^MS^ 


308 


THE  CSANUL  HCSCDLATCRE 


developmeat  of  the  branches  of  the  facial  nerve  which  supply  it 
(Fig.  12a). 

The  degeneration  of  the  upper  part  of  the  third  arch  produces  a 
shifting  forward  of  one  of  the  muscles  derived  from  its  bianchiomere, 
the  stylopharyngeus  arising  from  the  base  of  the  styloid  process. 
The  innervation  of  this  muscle  by  the  ninth  nerve  indicates,  however, 
ito  true  significance,  and  since  fibers  of  this  nerve  of  the  third  aich 
also  pass  to  the  constrictor  muscles  of  the  pharynx,  a  portion  of 
these  must  also  be  regarded  as  having  arisen  from  the  third 
branchiomere. 

The  cartilages  of  the  fourth  and  fifth  arches  enter  into  the  forma- 
tion of  the  larynx  and  the  muscles  of  the  corresponding  branchio- 
•  meres  constitute  the  muscles  of  the  larynx,  together  with  the  remain- 
ing portions  of  the  constrictors  of  the  pharynx  and  the  muscles  of 
the  soft  palate,  with  the  exception  of  the  tensor.  Both  these  arches 
have  branches  of  the  tenth  nerve  associated  with  them  and  hence 
this  nerve  suppUes  the  muscles  named.  In  addition,  two  of  the 
extrinsic  muscles  of  the  tongue,  the  glosso-palatinus  and  chon- 
droglossus,  belong  to  the  fourth  or  fifth  branchiomere,  although 
the  remaining  muscles  of  this  physiological  set  are  myotomic  in 
crigin. 

Finally,  portions  of  two  other  muscles  should  probably  be 
included  in  the  Ust  of  branchiomeric  muscles,  these  muscles  being 
the  trapezius  *.  i  stemomastoid.  It  has  ahtsady  been  seen  that 
they  are  partly  derived  from  the  cervical  myotomes,  but  they  are 
also  innervated  in  part  by  the  spinal  accessory,  and  since  the  motor 
fibers  of  this  nerve  are  serially  homologous  with  those  of  the  vagus 
it  would  seem  that  the  muscles  which  they  supply  are  probably 
branchiomeric  in  origin.  Observations  on  the  development  of 
these  muscles,  determining  their  relations  to  the  branchiomeres, 
are  necessary,  however,  before  their  morphological  significance  can 
be  regarded  as  definitely  settled. 

The  table  on  page  209  shows  the  relations  of  the  various  cranial 
muscles  to  the  myotomes  and  branchiomeres,  as  well  as  to  the  motor 
cranial  nerves. 


rax  CSANIAL  MracoiATOTi: 


ao9 


|l|!f|l||l|l|||| 


I  -^  I  •  ^  a  i  i  's  a 

ui  "  —     Bio.O'agg' 


■S   3 


iil 


THE   Uta  MUSCLES 


ii 


The  Limb  Muscles. — It  has  been  customary  to  regard  the  limb 
muscles  as  derivatives  of  certain  of  the  myotomes,  these  structures 
in  their  growth  ventrally  in  the  trunk  walls  being  supposed  to  pass 
out  upon  the  postaxial  surface  of  the  limb  buds  and  loop  back  again 
to  the  trunk  along  the  pneaxial  surface,  each  myotome  thus  giving 
rise  to  a  portion  of  both  the  dorsal  and  the  ventral  musculature  of 
the  limb.  This  view  has  not,  however,  been  verified  by  direct 
observation  of  an  actual  looping  of  the  myotomes  over  the  axis  of 
the  limb  buds;  indeed,  on  the  contrary,  the  limb  muscles  have  been 
found  to  develop  from  the  cores  of  mesenchyme  which  form  the 
axes  of  the  limb  buds  and  from  which  the  limb  skeleton  is  also 
developed.  This  may  be  explained  by  supposing  that  the  limb 
muscles  are  primarily  derivatives  of  the  myotomes  and  that  an 
extensive  concentration  of  their  developmental  history  has  taken 
place,  so  that  the  axial  mesenchyme  actually  represents  myotomic 
material  even  though  no  direct  connection  between  it  and  the 
myotomes  can  be  discovered.  Condensations  of  the  developmental 
history  certainly  occur  and  the  fact  that  the  muscles  of  the  human 
limb?  as  they  differentiate  from  the  axial  cores,  present  essentially 
the  same  arrangement  as  in  the  adult  seems  to  indicate  that  there  is 
actually  an  extensive  condensation  of  the  phylogenetic  history  of  the 
individual  muscles,  since  comparative  anatomy  shows  the  arrange- 
ment of  the  muscles  of  the  higher  mammaiian  limbs  to  be  the  result 
of  a  long  series  of  progressive  modifications  from  a  primitive  condi- 
tion. However,  even  though  this  be  the  case,  there  is  yet  the 
possibility  that  the  limb  musculature,  like  the  limb  skeleton,  may 
take  its  origin  from  the  ventral  mesoderm  and  consequently  belong 
to  a  different  embiyological  category  from  the  axial  myotomic 
muscles. 

The  strongest  evidence  in  favor  of  the  myotomic  origin  of  the 
limb  muscles  is  that  furnished  by  their  nerve  supply,  this  presenting 
a  distinctly  segmental  arrangement.  This  does  not  necessarily 
imply,  however,  a  corresponding  primarily  metameric  arrangement 
of  the  muscles,  any  more  than  the  pronouncedly  segmental  arrange- 
ment of  the  cutaneous  nerves  implie.  x  primary  metamerism  of  the 


IHB   UMB  MUSCLES 


311 


possesses  a  segmental,,  arranged  inner:.;;:,  tdTr;^,: 


supplied  by  a  deSf^  long'tudmally  along  the  bud  and  each 
spond,?g  treachtan/'^'"'  '.'  '"^^'-  '""'  f"^'h"™ore,  corre- 
limb  bud  he  eta  bi  •"",  ^'  ^""''''  ^P™^"'^')  ^"^^-^  "^  'he 
axial)  sJrflce  ,4ee  the  fib"™' v'^  '""'"^"^  "P°"  ">«  dorsal  (post- 
ooner  or  Uter  arrange  themselves  in  pneaxial  and  postanal 


Ill 


THE   UMB   MVSCUCf 


groups  as  Is  shown  in  the  diagram  Fig.  123.  The  first  nerve  which 
enters  the  limb  bud  lies  along  its  anterior  border,  and  consequently 
the  muscle  bands  which  are  supplied  by  it  will,  in  the  adult,  lie  along 


Fiti.  134. — External  Suxface  op  thz  Os  Imnominatuic  showing  thz  Attacbiiznt 

OF  Muscles  and  the  Zones  Supplied  by  the  Various  Nixves. 

la,  Twelfth  thoracic  nerve;  /  to  K,  lumbar  nerves;  i  and  a,  sacral  nerves. — (Bolk.) 

the  outer  side  of  the  arm  and  along  the  inner  side  of  the  leg,  in  conse- 
quence of  the  rotation  in  opposite  directions  which  the  limbs  undergo 
during  development  (see  p.  loi). 


THE    LIMB   MUSCIES 


»'3 


.ucce«,ve..  f.„  before  backward'  Jrlt^  lu^'JwTndT 


>V.TH  ™e  THOWC.C  Se«/s  -^X  »>■«*)'  "'  '^^"»"«^"  -N  CONm^Ax"; 

first  and  second  sacral  nerves.    The  arraneeraent  of  ,h 
bands  supplied  by  these  nerves  and  the  muscles Tfthe  ^I  to^^t 
-hey  contribute  may  be  seen  fn.m  Fig.  z.4.     mL  i  showntlri  i 
only  the  upper  portions  of  the  postaxial  bands,  their  lowlr  p^^ 


ai4 


THE  UMB  MUSCLES 


extending  downward  on  the  anterior  surface  of  the  leg.  Only  the 
sacral  bands,  however,  extend  throughout  the  entire  length  of  the 
limb  into  the  foot,  the  second  lumbar  band  passing  down  only  to 
about  the  middle  of  the  thigh,  the  third  to  about  the  knee,  the  fourth 
to  about  the  middle  of  the  cms  and  the  fifth  as  far  as  the  base  of  the 
fifth  metatarsal  bone,  and  the  same  is  true  of  the  corresponding 
prseaxial  bands,  which  descend  from  the  ventral  surface  of  the  os 
coxae  (innominatum)  along  the  inner  and  posterior  surfaces  of  the 
leg  to  the  same  points.  The  first  and  second  sacral  bands  can  be 
traced  into  the  foot,  the  first  giving  rise  to  the  musculature  of  its 


FlO.    H6. — SZCnOH  IHHOnOH  THI  UPPEB  PAET  OT  THE  AlK  SHOWDtO  THE  ZONES 

Supplied  by  the  Nesvxs. 
5v  to  7v,  Ventral  branches;  %d  to  Bd,  dois&l  branches  of  the  cervical  nerves. — (Bolk.) 

iimer  side  and  the  second  to  that  of  its  outer  side,  the  pneaxial  bands 
forming  the  plantar  musculature,  while  the  postaxial  ones  are  upon 
the  dorsum  of  the  foot  as  a  result  of  the  rotation  which  the  limb  has 
undergone. 

In  a  transverse  section  through  a  limb  at  any  level  all  the  muscle 
bands,  both  pneaxial  and  postaxial,  which  descend  to  that  level 
will  be  cut  and  will  lie  in  a  definite  succession  from  one  border  of  the 
limb  to  the  other,  as  is  seen  in  Fig.  125.  In  the  differentiation  of  the 
indi%'idual  muscles  which  proceeds  as  the  ner\'es  extend  from  the 
trunk  into  the  axial  mesenchyme  of  the  limb,  the  muscle  bands 


THE   LIMB   MUSCLES 


2IS 


undergo  modificaUons  similar  to  those  already  described  as  occurrinK 
m  the  trunk  myotomes.  Thus,  each  of  the  muscles  represented  in 
F>g.  125,  B,  IS  formed  by  the  fusion  c'  c.  .n.nfs  derived  from  two 
or  more  bands;  the  soleus  and  gast  ocnemius  re;.r.sent  deep  and 
superficial  layers  formed  from  the  ...o  bands  by  a  horizontal 
(tangential)  splittmg,  these  same  mu.  ...  ,o,.air,  a  portion  of  the 
second  sacral  band  which  overlaps  muscles  composed  only  of  higher 
myotomes,  and  the  intermuscular  septum  between  the  peroneus 
brevis  and  the  flexor  hallucis  longus  represents  a  portion  of  the  third 
sacral  band  which  has  degenerated  into  connective  tissue 

A  similar  arrangement  occurs  in  the  bands  which  are  to  be  recoe- 
mzed  m  the  musculature  of  the  upper  limb.  These  are  supplied  by 
the  fourth,  fifth,  sixth,  seventh  and  eighth  cenical  and  the  first 
thoracc  nerves,  and  only  those  supplied  by  the  eighth  cervical  and 
the  hrst  thoracic  nerves  extend  as  far  as  the  tips  of  the  fingers  The 
arrangement  of  the  bands  in  the  upper  part  of  the  brachium  may  be 
seen  from  Fig.  ,26,  in  connection  with  which  it  must  be  noted  that 
the  fourth  cervical  band  does  not  extend  down  to  the  level  at  which 
the  section  is  taken  and  that  the  pneaxial  band  of  the  eighth  cervical 
ner^'e  and  both  the  pneaxial  and  postaxial  bands  of  the  first  thoracic 
are  represented  only  by  connective  tissue  in  this  region 

In  another  sense  than  the  iongitudinal  one  there  is  a  division 
of  the  limb  musculature  into  more  or  less  definite  areas,  namely  in  a 
transverse  direction  in  accordance  with  the  jointing  of  the  skeleton 
Ihus,  there  may  be  recognized  a  group  of.muscles  which  pass  from 
the  axial  skeleton  to  the  pectoral  girdle,  another  from  the  limb 
girdle  to  the  brachium  or  thigh,  another  from  the  brachium  or  thigh 
to  the  antibrachium  or  crus,  another  from  the  antibrachium  or  cms 
to  the  carpus  or  tarsus,  and  another  from  the  carpus  or  tarsus  to  the 
digits.     This  transverse  segmentation,  if  it  may  be  so  termed,  is  not 
however,  perfectly  definite,  many  muscles,  even  in  the  lower  verte- 
brates, passing  over  more  than  one  joint,  and  in  the  mammalia 
especially,  it  is  further  obscured  by  secondary  migrations,  by  the 
partial  degeneration  of  muscles  and  by  an  end  to  end  union  of 
pnmanly  distinct  muscles. 


.■  k 


IHL  il 


3l6 


THE    LIHB    HUSCIES 


The  latissimus  dorsi,  serratiis  anterior  and  pectoral  muscles  are 
all  examples  of  a  process  of  migration  as  is  shown  by  their  innervation 
from  cervical  nerves,  as  well  as  by  the  actual  migration  which  has 
been  traced  in  the  developing  embryo  (Mall,  Lewis).  In  the  lower 
limb  evidences  of  migration  may  be  seen  in  the  femoral  head  of  the 
biceps,  comparative  anatomy  showing  this  to  be  a  derivative  of  the 
gluteal  set  of  muscles  which  has  secondarily  become  attached  to  the 
femur  and  has  associated  itself  with  a  prsaxial  muscle  to  form  a 
compound  structure.  An  appearance  of  migration  may  also  be 
produced  by  a  muscle  making  a  secondary  attachment  below  its 
original  origin  or  above  the  insertion  and  the  upper  or  lower  pvt. 
as  the  case  may  be,  then  degenerating  into  connective  tissue.  This 
has  been  the  case  with  the  peroneus  longus,  which,  in  the  lower 
mammals,  has  a  femoral  origin,  but, has  in  man  a  new  origin  from 
the  fibula,  its  upper  portion  being  represented  by  the  fibular  lateral 
ligament  of  the  knee-joint.  So  too  the  pectoralis  minor  is  primarily 
inserted  into  the  humerus,  but  it  has  made  a  secondary  attachment 
to  the  coracoid  process,  its  distal  portion  forming  a  t  oraco-humeral 
ligament. 

The  comparative  study  of  the  flexor  muscles  of  the  antibrachial 
and  crural  regions  has  yielded  abundant  evidence  of  extensive 
modifications  in  the  differentiation  of  the  limb  muscles.  In  the 
tailed  amphibia  these  muscles  are  represented  by  a  series  of  super- 
posed layers,  the  most  superficial  of  which  arises  from  the  humerus 
or  femur,  while  the  remaining  ones  take  their  origin  from  the  ulna 
or  fibula  and  are  directed  distally  and  laterally  to  be  inserted  either 
into  the  palmar  or  plantar  aponeurosis,  or,  in  the  case  of  the  deeper 
layers,  into  the  radius  (tibia)  or  carpus  (tarsus).  In  the  arm  of  the 
lower  mammalia  the  deepest  layer  becomes  the  pronator  quadratus, 
the  lateral  portions  of  the  superficial  layer  are  the  flexor  carpi  ulnaris 
and  the  flexor  carpi  radialis,  while  the  intervening  layers,  together 
with  the  median  portion  of  the  superficial  one,  assuming  a  more 
directly  longitudinal  direction,  fuse  to  form  a  common  flexor  mass 
which  acts  on  the  digits  through  the  palmar  aponeurosis.  From 
this  latter  structure  and  from  the  carpal  and  metacarpal  bones  five 


THE    LIMB    MUSCLES 


217 


layers  of  palmar  muscles  take  origin.    The  radial  and  ulnar  portions 
of  the  most  superficial  of  these  become  the  flexor  pollicis  bre^is Id 

of  the  layer  degenerates  mto  connective  tissue,  forming  tendons 


^'^^^'YnrZTZ^^Z':7^r^t^^^^  -"  f^)  .*e  hand  si.„.i„, 

hon«n.afiy,  the  second  byer  veMiValL  U,T  S  5'  „hTi^  f  Pf'*™!  lay"  is  shaded 
verlicaUy,  and  Ihe  fifth  obliquelyto  the  ri4t  jwf  ^f!''  '°  "5?  ''"•  *'  '"""h 
adductor  poUicis;  BR.  brachio-radWh  ij-^  '  xfe^^'r  l-'J""  '''8'"  ''"'""•  '''"'. 
extensorcarpiulnaris;£/,exten5orSi,'fw/)       f^        Jgilprum  communU;  ECU, 

flexor  carpi  ulnaris;  i^i/.,  flexor  3>WAiinJ^TFui',^'T ■'"'''  "'''""'■  "f"'. 

which  pass  to  the  four  ulnar  digits.     Gradually  superficial  portions 
layers  of  the  palmar  aponeurosis  from  which  the  tendons  representing 


V: 


2l8 


TBE    UMB   MUSCLES 


the  superficial  layer  of  the  palmar  muscles  arise,  and  they  form  with 
these  the  flexor  digitorum  sublirais.  The  deeper  layers  of  the  anti- 
brachial  flexor  mass  become  the  flexor  digitorum  profundus  and 
the  flexor  pollicis  longus  (Fig.  127,  A),  and  retain  their  connection 
with  the  deeoer  layers  of  the  palmar  aponeurosis  which  form 
their  tendonsi  and  since  tl.e  second  layer  of  the  palmar  muscles 
takes  origin  from  this  portion  of  the  aponeurosis  it  becomes  the 
lumbrical  muscles,  arising  from  the  profundus  tendons  (Fig.  127, 


Fig  128  —Transverse  sections  through  (A)  the  eras  and  (B)  the  foot,  shovvmg  the 
arrangement  of  the  layers  of  the  fleior  muscles.  The  shading  has  the  same  sigmficanc. 
as  in  the  preceding  figure.  AbH,  abductor  haUucis;  AbM,  ahductor  mmum  digit.; 
AdH,  adductor  hallucis;  ELD,  extensor  longus  digitoram;  F,  fibula;  FBD,  fleior 
brevis  digitorium;  FBH,  flexor  brevis  hallucU;  FBM,  flexor  brevis  minum  digiti; 
PLD,  flexor  longus  digitorum;  G,  gastrocnemius;  ID,  interossei  dorsalis;  IV,  mterossei 
ventrales;  L,  lumbricales;  P,  pUntuis;  Pe,  peroneus  longus;  Po,  popliteus;  S,  loleus; 
7',  tibia;  TA,  tibialis  anticus;  TP,  tibiars  posticus;  I-V,  first  to  fifth  metautsal. 

B).  The  third  layer  of  palmar  muscles  becomes  the  adductors 
of  the  digits,  reduced  in  man  to  the  adductor  pollicis,  while  from 
the  two  deepest  layers  the  interossei  are  developed.  Of  these 
the  fourth  layer  consists  primarily  of  a  pair  of  slips  correspond- 
ing to  each  digit,  while  the  fifth  is  represented  by  a  series  of  muscles 
which  extend  obliquely  across  between  adjacent  metacarpals. 
With  these  last  muscles  certain  of  the  fourth  layer  slips  unite  to  furra 
the  dorsal  interossei,  while  the  rest  become  the  volar  interosi<!i. 
The  modifications  of  the  almost  identical  primary  arrangement 
in  the  crus  and  foot  are  somewhat  different.     The  superficial  layer 


UTEKATUSE 


319 


of  the  crural  flexors  becomes  the  gastrocnemius  and  plantaris  (Fig. 
128,  A)  and  the  deepest  layer  becomes  the  popliteus  and  the  inter- 
osseous membrane.  The  second  and  third  layers  unite  to  form  a 
common  mass  which  is  inserted  into  the  deeper  layers  of  the  plantar 
aponeurosis  and  later  differentiates  into  the  soleus  and  the  long 
digital  flexor,  the  former  shifting  its  insertion  from  the  plantar 
aponeurosis  to  the  os  calcis,  while  the  flexor  retains  its  connection 
with  the  deeper  layers  of  the  aponeurosis,  these  separating  from  the 
superficial  layer  to  form  the  Jong  flexor  tendons.  The  fourth  layer 
partly  assumes  a  longitudinal  direction  aud  becomes  the  tibialis 
posteiior  and  the  flexor  hallucis  longus  and  partly  retains  its  original 
oblique  direction  and  its  connection  with  the  deep  layers  of  the 
plantar  aponeurosis,  becoming  the  quadratus  plantae.  In  the  foot 
(Fig.  128,  B)  the  superficial  layer  persists  as  muscular  tissue,  forming 
the  abductors,  the  flexor  digitorum  brevis  and  the  medial  head  of  the 
flexor  hallucis  brevis,  the  second  layer  becomes  the  lumbricales,  and 
the  third  the  lateral  head  of  the  flexor  hallucis  brevis  and  the  adduc- 
tor hallucis,  while  the  fourth  and  fifth  layers  together  form  the  inter- 
ossei,  as  in  the  hand,  the  flexor  quinti  digiti  brevis  really  belonging 
to  that  group  of  muscles. 


•  ft    'I 


LITERATURE. 

C.  R.  Bakdeen  and  W.  H.  Lewis:  "Development  of  the  Limbs,  Body-wall,  and 

Back  in  Man,"     The  AmerKan  Journal  of  Anai.,  I,  1901. 
K.  Baxdeleben:  "Musiiel  und  Fascia,"   Jmaiscke  Zeilschr.  far  Nmurmssmsch., 

XV,  18X3. 
L.  Bolk:  "  Reziehungen  zwischen  Skelctt,  Muskulatur  und  Nerven  der  Eitremitaten, 

dai^legt  am  Beckengtirtel,  an  dessen  Muskulatur  sowie  am  Plexus  lumbo- 

sacralis,"  Morphol.  Jahrbuch,  XXI,  1894. 
L.  Bolk:  "  Rekonstruktion  der  Si;gmentirung  olt  Gliedmassenmuskulatur  dargelegt 

an  den  Muskeln  des  Oberechenkels  und  des  Schultergttrtels,"  Morfhol.  Jahrbuch, 

XXII,  1895. 
L.  Bolk:  "  Die  Sklerozonie  des  Humerus,"  Morphol.  Jahrbuch,  xxiii,  1896. 
L.  Bolk:  "Die   Segraentaldifferenzierung  des  menschlichen   Rumpfes  und  seiner 

Extremitaten,"  i,  Morphol.  Jahrbuch,  xxv,  1898. 
K.  Futaiilka:  "Ueber  die  Entnickelung  der  Facialisrauskulatur  des  Menschen," 

Artti.  Hfft^,  XXX,  1906. 
E.  Godlewski:  "Die  Entwickluiig  des  Skelet-  und  Herziauskelgewebes  -ler  Stluge- 

thjere,"  Archh'/ar  mikr.  Anat.,  LX,  190a, 


230 


UTERATU&E 


E.  GRiCnENBERG:  "Die  Entwicklung  der  menschUchen  Beckramuikulatur/'  Anat. 

Hefte,  XXIII,  1904. 
W.  F.  HsRRiNGHAif:  "The  Minute  Anatomy  of  the  Brachial  Plexus,"  Proceedings 

0/ the  Royal  Soc.  London ,  xli,  1886. 
W.  H.  Lkwis:  "ThcDevebpmentof  the  AnninMan," /liiMr./ottr.^i4fu/.,i,  1903 
J.  B.  MacCallum:  "On  the  Hbtology  and  Histogenesis  of  the  Heart  Muscle-cell/' 

Anat.  Anseiger,  xiii,  1897. 
J.  B.  MAcCALLtnc:  "On   the   Histogenesis   of   the   Striated   Muscle-fiber  and  the 

Growth  of  the  Human  Sartorius  Muscle,"  Jolms  Hopkins  Hospital  Bulletin,  1890 

F.  P.  Mall:  "Development  of  the  Ventral  Abdominal  Walls  in  Man,"  Joum.  0/ 

Morphol.,  XIV,  1898. 
Caroline  McGill:  "The  Histogenesis  of  Smooth  Muscle  In  the  Alimentary  Canal 
and  Respiratory  Tract  of  the  Pig,"  Internal.  Monatschr.  Anat.  und  Phys.,  xxiv, 
1907. 

The  Phylogeny  of  the  Forearm  Flexors,"  Anur.  Joum,  of  Anat., 


"The  Phylogeny  of  the  Palmar  Musculature,"  Amer.  Joum.  of 
"The  Phylogeny  of  the  Crural  Flexors,"  Amer.  Joum.  of  Anat., 
"The  Phylogeny  of  the  Plantar  Musculature,"  Amer.  Joum.  of 


J.  P.  McMurrich: 

n,  1903. 
J.  P.  McMurrich: 

Anai.,  n,  1901 
T.  P.  McMurricb: 

IV,  1904. 
J   P.  McMurrich:  ' 

Anat.,  VI,  1907. 

A.  Meek:  "  Preliminary  Note  on  the  Post-embryonal  History  of  Striped  Muscle-fibers 

in  Mammalia,"  Anat.  Anteiger,  xiv,  1898.    (See  also  Anat.  Anteiger,  xv,  1899.) 

B.  MoRPURGo:  "Ueber  die  post-embiyonale  Entw.'-kelungderquergestreiftenMuskel 

von  weissen  Ratten,"  Anat.  Anteiger,  xv,  1899. 
I.  FopowsKY:  "  Zur  Entwicklungsgescfaichte  des  N.  facialis  beim  Menschen,"  Jlfor^W. 

Jakrbuch,  xxiif,  1896. 
1.  Popowsky:  "  Zur  Entwickelungsgeschichte  der  Dammmuskulatur  beim  Menscben," 

Anat,  Hefte,  xi,  1899. 
L.  Rethi:  "Der  peripheren  Verlauf  der  motorischen  Rachen-  und  Gaumennerven," 

Sitsungsber.  der  kais.  Akad.  Wissensch.  Wien.  Math.-Naturwiss.  Classe,  Cii,  1893. 

C.  S.  Sherrington:  "  Notes  on  the  Arrangement  of  Some  Motor  Fibers  in  the  Lumbo- 

sacral Plexus,"  Journal  of  Physiol.,  xiii,  1892. 
J.  B.  Sutton:  "Ligaments,  their  Nature  and  Morphology,"  London,  1897. 


CHAPTER  IX. 

THE  DEVELOPMEHT  OF  THE  CIRCULATORY  AND  LYM- 
PHATIC SYSTEMS. 

At  present  nothing  is  known  as  to  the  earUest  stages  of  develop- 
ment of  the  circulatory  system  in  the  human  embryo,  but  it  may  be 
supposed  that  they  resemble  in  their  fundamental  features  what  has 
been  observed  in  such  forms  as  the  rabbit  and  the  chick.  In  both 
these  the  system  originates  in  two  separate  parts,  one  of  which, 
located  m  the  embryonic  mesoderm,  gives  rise  to  the  heart,  while  the 
other,  ansmg  in  the  extra-embryonic  mesoderm,  forms  the  first 
blood-vessels.  It  will  be  convenient  to  consider  these  two  parts 
separately,  and  the  formation  of  the  blood-vessels  may  be  first 
described. 

In  the  rabbit  the  extension  of  the  mesoderm  from  the  embryonic 
region,  where  it  first  appears,  over  the  yolk-sac  is  a  gradual  process, 
and  It  is  in  the  more  peripheral  portions  of  the  layer  that  the  blood- 
vessels first  make  their  appearance.  They  can  be  distinguished 
before  the  splitting  of  the  mesoderm  has  been  completed,  but  are 
always  developed  in  that  portion  of  the  layer  which  is  most  intimately 
associated  with  the  yolk-sac,  and  consequently  becomes  the  splanch- 
nic layer.  They  belong,  indeed,  to  the  deeper  portion  of  that  layer 
that  nearest  the  endoderm  of  the  yolk-sac,  and  so  characteristic  is 
their  origin  from  this  portion  of  the  layer  that  it  has  been  termed  the 
angtoblast  and  has  been  held  to  be  derived  from  the  endoderm 
independently  of  the  mesoderm  proper.  The  first  indication  of 
blood-vessels  is  the  appearance  in  the  peripheral  portion  of  the 
mesoderm  of  cords  or  minute  patches  of  spherical  cells  (Fip  129,  A). 
These  increase  in  size  by  the  division  and  separation  of  the  cells  from 
one  another  (Fig.  129,  B),  a  clear  fluid  appearing  in  the  intervals 
which  separate  them.    Soon  the  cells  surrounding  each  cord  arrange 


333  DEVELOPMENT   OF   THE   BLOOD-VESSELS 

themselves  to  form  an  enclosing  wall,  and  the  cords,  increasing  in 
size,  unite  together  to  form  a  network  of  vessels  in  which  float  the 
spherical  cells  which  may  be  known  as  mesamaeboids  (Minot). 
Viewed  from  the  surface  at  this  stage  a  portion  of  the  vascular  area 
of  the  mesoderm  would  have  the  appearance  shown  in  Fig.  130, 
revealing  a  dense  network  of  canals  in  which,  at  intervals,  are 
groups  of  mesamxboids  adherent  to  the  walls,  constituting  what  have 
been  termed  the  blood-islands,  while  in  the  meshes  of  the  network 
unaltered  mesoderm  cells  can  be  seen,  forming  the  so-called  sub- 
stance-islands. . 


Fro.  lag. — ^Transverse  Section  through  the  Area  Vasculosa  of  Rabbit 
eubryos  showing  the  transformation  of  mesoderm  cells  into  the  vascular 
Cords. 

Ec,  Ectoderm;  En,  endoderm;  Me,  mesoderm. — {van  der  Stricht.) 

At  the  periphery  of  the  vascular  area  the  vessels  arrange  them- 
selves to  form  a  sinus  terminalis  enclosing  the  entire  area,  and  the 
vascularization  of  the  splanchnic  mesoderm  gradually  extends 
toward  the  embryo.  Reaching  it,  the  vessels  penetrate  the  embry- 
onic tissues  and  eventually  come  into  connection  with  the  heart, 
which  has  already  differentiated  and  has  begun  to  beat  before  the 
coimection  with  the  vessels  is  made,  so  that  when  it  is  made  the 
circulation  is  at  once  established.  Before,  however,  the  vasculariza- 
tion reaches  the  embryo  some  of  the  canals  begin  to  enlarge  (Fig. 


DEVELOPMENT  OF   THB    BIOOD-VESSEIS  3ij 

i3hA),  producing  arteries  and  veins,  the  rest  of  the  network  forming 
cap,l  anes  nn.f  ng  these  two  sets  of  vessels,  and,  this  process  continu 
ng,  there  ,.re  eventually  differentiated  a  single  vilellin.  arUry  and 
two  vueUtne  veins  (Fig.  131,  B). 

In  the  human  embryo  the  small  size  of  ine  yolk-sac  permits  of  the 
extension  of  the  vascular  area  over 
its  entire  surface  at  an  early  period, 
and  this  condition  has  already  been 
reached  in  the  earliest  stages  known 
and  consequently  no  sinus  termin- 
alis  such  as  occurs  in  the  rabbit  is 
visible.  Otherwise  the  conditions 
are  probably  similar  to  what  has 
been  described  above,  the  first  cir- 
culation developed  being  associated 
with  the  yolk-sac. 

It  is  to  be  noted  that  the  capil- 
lary network  of  the  area  vasculosa 
consists  of  relatively  wide  anasto- 
mosing spaces   whose  endothelial 
lining  rests  directly  upon  the  sub- 
stance islands  (Fig.  130).    In  cer- 
tain of  the  embryonic  organs,  not- 
ably  the    liver,   the  metanephros 
and  the  heart,  the  network  has  a 
similar  character,  consisting  of  wide 
anastomosing  spaces  bounded  by  v!i?^.'^-^l^  ^^^^  o'i 
an  endothehum    which  rests   di-   *  *^<^*- 
rectly,  or  almost  so,  upon  the  par-   by'^ri^ JTrS-n"  'if"^. 
enchyina  of  the  organ  (the  hepatic   '^^^''  ^''  ™'»""'«-'siand.-(Dt,„.) 
cylinders,  the  mesonephric  tubules,  or  the  cardiac  muscle  trabecute) 
(F.gs.  132  and  190,  B).    To  this  form  of  capillary  the  term  sinusoid 

^n^T^I  ?"°'^'  '"'^  "  ^PP^''^ '°  '^  '--<»  by  the  e^ 
Zu    V    ,7  "  P''^'°"''y  ^^■^""g  blood-vessel,  which  thus 

moulds  .tself,  as  it  were,  over  the  parenchyma  of  the  organ.    The 


334 


THE   rOUIATION  OF  THE   BUMO 


true  capillaries,  on  the  other  hand,  are  more  definitely  tubular  in 
form,  are  usually  imbedded  in  mesenchymatous  connective  tissue 
and  are  developed  in  the  same  manne.  as  the  primary  capillaries 
of  the  area  vasculosa,  by  the  aggregation  of  vasifactive  cells  to  form 
cords,  and  the  subsequent  hollowing  out  of  these.  Whether  these 
vasifactive  cells  are  new  differentiations  of  the  embryonic  mesen- 
chyme or  are  budded  off  from  the  walls  of  existing  capillaries  which 
have  grown  in  from  extra-embryonic  regions,  is  at  present  undecided. 
The  Fomuttlon  of  the  Blood.— The  mesamoeboids,  which  are 


Fio.  131.— Tb«  Vastolax  Auas  of  Rabbit  Embryos.  In  B  thi  Veins  au 
Kepbisintid  by  Black  and  thi  Nktwobk  is  OmmB.—hrm  Bmtdm  md 
Julm.) 

the  first  formed  blood-corpuscles  are  all  nucleated  and  destitute  or 
nearly  so  of  hemoglobin.  They  have  been  held  by  some  observers 
to  be  the  only  source  of  the  various  forms  of  corpuscles  that  are 
found  in  the  adult  vessels,  while  others  maintain  that  they  give  rise 
only  to  the  red  corpuscles,  the  leukocytes  arising  in  tissues  external 
to  the  blood-vessels  and  only  secondarily  making  their  way  into 
them.  According  to  this  latter  view  the  red  and  white  corpuscles 
have  a  different  origin  and  remain  distinct  throughout  life. 


IHE  rowiAnoN  or  the  blood  „j 

So  long  as  the  formation  of  blood-vessels  U  t«tm„  ..i       •     ^ 

"■"  "le  emoryonic  mesoderm  or  merclv  naw  inf«  .k.       u 
region  from  the  more  peripheral  ar^as     i?!!!  1     embryonic 

time  of  birth.    It  must  be  remembered,  however  fh»t  »!,/./     . 

corpuscle    ^d  for    h/       .         T^  ^"^  '^^  ^°™»"°»  °f  »ew 
sCs  of  life  '  ""'''""  °'  hematopoietic  organs  at  all 

ay  pass  and  so  come  mto  mUmate  relations  with  the  actual  tissues 


9a6 


m  rouiATioN  or  the  blood 


of  the  organs  (Fig.  13a).  After  birth  the  haematopoietic  function  of 
the  liver  ceases  and  that  of  the  spleen  becomes  limited  to  the  forma- 
tion of  whitr  corpuscles,  though  the  complete  function  may  be 
re-established  in  cases  of  extreme  anemia.  The  bone-marrow, 
however,  retains  the  function  completely,  being  throughout  life  the 
seat  of  formation  of  both  red  and  white  corpuscles,  the  lymphatic 
nodes  and  follicles,  as  well  as  the  spleen,  assisting  in  the  formation 
of  the  latter  elements. 

The  mesamoeboids  early  become  converted  into  nucleated  red 

corpuscles  or  erythrocytes  by 
the  development  of  hemoglo- 
bin in  their  cytoplasm,  their 
nuclei  at  the  same  lime  be- 
coming granular.  Up  to  a 
stage  at  which  the  embryo  has 
a  length  of  about  13  mm.  these 
are  the  only  form  of  red  cor- 
puscle in  the  circulation,  but 
at  this  time  (Minot)  a  new 
form,  characterized  by  its 
smaller  size  and  more  deeply 
staining  nucleus,  makes  its  ap- 
pearance. These  erythrocytes 
have  been  termed  normoblasts 
(Ehrlich),  although  they  are 
merely  transition  stages  lead- 
ing to  the  formation  of  erythro- 
plastids  by  the  extrusion  of  their  nuclei  (Fig.  133).  The  cast-off 
nuclei  undergo  degeneration  and  phagocytic  absorption  by  the 
leukocytes,  and  the  masses  of  cytoplasm  pass  into  the  circulation, 
becoming  more  and  more  numerous  as  development  proceeds, 
until  finally  they  are  the  typical  hsemoglobin-containing  elements 
in  the  blood  and  form  what  are  properly  termed  the  red  blood 
corpuscles 

It  has  already  (p.  334)  been  pointed  out  that  discrepant  views 


FlO.     133. — SlCnON    OF    A    POKTIOH     OF 
IHX  LiVXR  OF  A  RABBrr  EllBKYO  OF  5  UU. 

e,  Erythrocytes  in  the  liver  sutistaiice  and 
in  &  capillary;  k,  hepatic  cells. — (van  dtr 
SirldU.) 


IH«  F0«IIAnON  OF  THE   BLOOD  „. 

prevail  as  to  the  origin  of  the  white  blood  -rorpuKlcs     Indeed  ihr,. 

one  view  they  have  a  common  origin  with  the  erythrocytes  from  the 
fTom  1  .  (^"■'T'^")'  --ding  to  ano.L  theyTreZnt 
rom  mesenchyme  cells  outside  the  cavities  of  the  blood^vS 
(Max.mow)  while  according  to  a  third  view  the  first  for^  S£. 

collect  in  the  lymphoid  tissues,  such  as  "><=  I'vdmg  cells  to 

the  lymph  nodes,  tonsils,  etc.,  to  form  /fi\  |^    ^.    A      « 

more  or  less  definite  groups  which  ^  W  ^  <^  P) 
have  been  termed  germ-centers  (Flem-  ^^ 

ming).    The  new  cells  when  they  first  t'''°-   '33 -Si*om  ih  th» 

pass  into  the  circulation  have  a  rel-  -VS«"ir  ^J  ^.v^"^ 

atively  large  nucleus  surrounded  by  a  '"""'— '""" ''"•  ^cfe*/) 

thTlrr'  °i  '^TT  *'""'"'  ""'""'^ ''"'»'  ^'""  'hey  resemble 
Ae  cells  found  ,n  the  lymphatic  vessels,  are  termed  lymfHocyUs 
(F'g.  134,  a).  In  the  orcuUtion,  however,  other  forms  of  leukocytes 
al^  occur,  which  are  believed  to  have  their  origin  from  cells  wth 

Zl^Tyr'^irf""''  '"^"-^''"'  =y'°P'-".  which  occur 
h^ughout  hfe  m  the  bone-marrow  and  have  been  termed  myelo- 

wht^\.  J  t""'I".  ''^'  ^'''  '"  '^'  circulation,  constitute 

It-  h    M  ifT''  ''"""''''  '"''"'>'"  ("^'rophile  cells  of 

Ehrhch)  (F,g  134,  J),  but  whether  these  and  the  myelocytes  are 
denved  from  lymphocytes  or  have  an  independent  orL  iVas  Z 
undetermined.  Less  abundant  are  the  coarsely  granZ llkoclus 
(.eos^nopkUe  cells  of  Ehrlich)  (Fig.  X34.  c),  charactered  by  the  ctS: 
ness  and  staming  reactions  of  their  cytoplasmic  granules  and  by 

ttv«  T!l°"^  °li  '""''"''"'^  °"'^'^"=-  '^^'y  ^"^  Probably  deriva- 
Uves  of  the  finely  granular  type  and  it  has  been  maintained  by 
We,denre,ch  that  their  granules  have  been  acquired  by  the  phago- 
cytosis  of  degenerated  erythrocytes.  FinaUy,  a  third  type  is  formed 
by  the  polymorphonuclear  or  polynudear  leukocytes  (basophiU  ceUs 


338 


THX   FOUIATIOK  OF   THE   BLOOD 


of  Ehrlich)  (Fig.  134,  d),  which  are  to  be  regarded  as  leukocytes  in 
the  process  of  degeneration  and  are  characterized  by  their  irregu- 
larly lobed  or  fragmented  nuclei,  as  well  as  by  their  staining 
peculiarities. 

In  the  fetal  hematopoietic  organs  and  in  the  bone-marrow  of  the 
adult  large,  so-called  gianl-cells  are  found,  which,  although  they  do 
not  enter  into  the  general  circulation,  are  yet  associated  with  the 
development  of  the  blood-corpuscles.    These  giant-cells  as  they 


Fio.  134. — ^FiGuus  or  the  DirnxENT  Fouis  of  Whue  Corpuscles  occcuiNa 

IN  HuHAH  Blood. 

0,  Lymphocytes;  b,  findy  granular  (neutrophUe)  leukocj  ,e;  c,  coarsely  granular  (eosino- 

phile)  leukocyte;  d,  polymorphonuclear  (basophile)  leukocyte.— (ITwifelirnc*.) 

occur  in  the  bone-marrow  are  of  two  kinds  which  seem  to  be  quite 
distinct,  although  both  are  probably  formed  from  leukocytes.  In 
one  kind  the  cytoplasm  contains  several  nuclei,  whence  they  have 
been  termed  polycaryocytes,  and  they  seem  to  be  the  cells  which  have 
ateady  been  mentioned  as  osteoclasts  (p.  158).  In  the  other  kind 
(F'8-  ^iS)  the  nucleus  is  single,  but  it  is  large  and  irregular  in  shape, 
frequently  appearing  as  if  it  were  producing  buds.  These  mega- 
caryocytes  appear  to  be  phagocytic  cells,  having  as  their  function  the 
destruction  of  degenerated  corpuscles  and  of  the  nuclei  of  the 
erythrocytes. 


THE  FOMCATION  OP  THE  HEABT  „p 

7  give  nse  in  tlieir  amoeboid  movements  (Fig.  135). 


lii"^^^'^'^^°^0.^^\f^^^-^^Z,^'>    EX^KO.0     TWO 
OFF  BlOOD-PUTElETS.  "  °'  "LOOD-VESSIL  AND  IS  BdSDINO 

Of,  Blood-platdeb;  V.  blood-v«sel.-(7.  H.  Wrigk,.) 

wh,?!t  ^''T"*^"  "^  *^«  Heart.-The  heart  makes  its  appearance 
while  the  embryo  is  still  spread  out  upon  the  surface  of  the^lk  ^c 

toct  in  the  median  line.  On  each  side  of  the  body  near  the  manrins 
of  the  embryom-c  area  a  fold  o!  the  splanchnoplLrippLrTr^ 
J^jng  mto  the  cclomic  cavi.y,  and  within  this  fod  aTi^'C" 

Tl(lor^'/^\  fti°^'^  ^'"  P''^"^^  "  P«rti°°  of  the  muscular 

P^eld,    h!  ;    ^  u  '^°'^'"^"°"  °f  *=  en^bryo  from  the  yolk-sac 

I  nf^;  ?  .r  "^^  "'  ^^'^^'''y  ''~"8''t  "«■•"  together  (pTg 
36,  B),  until  they  meet  in  the  mid-vential  line,  when  the  my,^  fi 
folds  and  endocardial  sacs  fuse  together  (Fig.  1,6  oTo  W  . 
cyhndncal  heart  l>nng  in  the  mid-ventral  iLe'of  £  SiJ  il^nt 
of  the  antenor  surface  of  the  yolk-sac  and  in  what  wiXr  bfS 


ii 


'3° 


THE   FOBMATION  OF  THE  HEAKT 


cervical  region  of  the  body.    At  an  early  stage  the  various  veins 
which  have  already  been  formed,  the  vitellines,  umb'licab,  jugulars 


OT    TBE     HXAKT 


FlO      ll6.— DlAOMAMS    iLlDSTIATmO    Tfflt    FO»KATION 
GUINZA-FIO. 

The  mesoderm  is  represented  in  black  and  the  endocardium  by  a  broken  line. 
am.  Amnion;  w,  endoderm;  »,  heart;  »,  digestive  tract.— M/fer  SirM  ami 
Cornu.) 

and  cardinals,  unite  together  to  open  into  a  sac-like  structure,  the 
smus  vmosus,  and  this  opens  into  the  posterior  end  of  the  heart 
cylinder.    The  anterior  end  of  the  cylinder  tapers  off  to  form  the 


THE   FOBIIATION  OF  THE  HEART  ajj 

ph«yngeal  region  and  carries  the  blood  away  from  the  heart     The 

Ji^out  i"^?  T'- "'°  ^  P°^'^"°^  '"^'»  °^  '••^  ^-«  '""e  Ind 
news  out  from  its  anterior  end. 

The  simple  cylindrical  form  soon  changes,  however,  the  heart 
mbem  embryos  of  ,.5  mm.  ,•„  length  having  become  bent  u^ 

the  left  IS  the  end  into  which  the  sinus  venosus  opens,  and  from  this 


Fio.  137.— Hea«t  o?  Ebbeyo  or  Pio    ..»— K,A..r  „    ij 

2  15   Mf.,    noH  A  RK»NST»DCnOK  .    » 10.    I38.— Hea»T   OF  ElIBIYO   OF 

..Atrium;  ri,  «ST.™TS1  tuJJS;  '""   "°"  ™'  ^'^'■ 

f,  TOitricIe;  vi,  jugular  vein"  ti  um '  /^.  Ductus Cuvien;  lA,  leftatiium 

bilid  YBnl-fHisy  '     •  'A:  "«•"  »•?""'; "/.  i-gul"  vein;  VI. 

lot  ventnde;  vu,    umbiUcal    vein.— 
(.Ba.) 

Sh^^  .V'"^,'''"  ""•*'"y  "'^•^  "•  ">«  "8ht,  where  it  again 
bends  at  fot  do^ally  and  then  anteriorly  to  pass  over  into  the  aLc 

•  H    »■  T5' P""'"'^  °f  *«  =""«  "Web  lies  dorsally  and  to  the  left 

ll?  '°w*     *""'  "''  '°  ^'^  '"^'''  ^"^  P°"'°"  ^^^^  passes  from 
nght  to  left  represents  the  future  left  ventricle,  while  the  succeeding 

TZ  "'^TT  '^'  "■«''  ''*°'^^'^-    ^  ''''"  ^'-^'s  (Fig.  X385 
the  left  ventricular  portion  drops  downward  in  front  of  the  atria 


'!f 


»3» 


THE   FOMtAnON  OF  THE  HEAKT 


portion,  asstuning  a  more  horizontal  position,  while  the  portion 
which  represents  the  right  ventricle  is  drawn  forward  so  as  to  lie  in 
the  same  plane  as  the  left. 

At  the  same  time  two  small  out-pouchings  develop  from  the 
atrial  part  of  the  heart  and  form  the  first  indications  of  the  two 
atria.  As  development  progresses,  these  increase  in  size  to  form 
large  pouches  opening  into  a  common  atrial  canal  (Fig.  139)  which 
is  directly  continuous  with  the  left  ventricle,  and  as  the  enlarge- 
ment of  the  pouches  continues  their  openings  into  the  canal  enlarge, 

until  finally  the  pouches  become 
continuous  with  one  another, 
forming  a  single  large  sac,  and 
the  atrial  canal  becomes  reduced 
to  a  short  tube  which  is  slightly 
invaginated  into  the  ventricle 
(Fig.  140). 

In  the  meantime  the  sinus 
venosus,  which  was  originally  an 
oval  sac  and  opened  into  the 
atrial  canal,  has  elongated  trans- 
versely until  it  has  assumed  the 
form  of  a  crescent  whose  convex- 
ity is  in  contact  with  the  walls  of 
the  atria,  and  its  opening  into  the 
heart  has  verged  toward  the  right,  until  it  is  situated  entirely  within  the 
area  of  the  right  atrium.  As  the  enlargement  of  the  atria  continues, 
the  right  horn  and  median  portion  of  the  crescent  are  graduaUy  taken 
up  into  their  walls,  so  that  the  various  veins  which  originally  opened 
into  the  sinus  now  open  directly  into  the  right  atrium  by  a  single 
opening,  guarded  by  a  projecting  fold  which  is  continued  upon  the 
roof  of  the  atrium  as  a  muscular  ridge  known  as  the  sepum  spurvum 
(Fig.  140,  sp).  The  left  horn  of  the  crescent  is  not  taken  up  into 
the  atrial  wall,  but  remains  upon  its  posterior  surface  as  an  elongated 
sac  forming  the  coronary  sinus. 

The  division  of  the  now  practicaUy  single  atrial  cavity  into  tht 


Fio.  139.— Heart  or  Embryo  of  5 
HM.,  Seen  p»oii  in  Front  an-j  sliohtiy 
raoM  Above.— (Hi's). 


THE   FOHHATION   OF   THE   HEART 


>33 


permanent  nght  and  left  atria  begins  with  the  formation  of  a  falci- 
form ndge  running  dorso-ventrally  across  the  roof  of  the  cavity 
This  IS  the  atrial  septum  or  septum  primum  (Fig.  140,  ss),  and  it 
rapidly  increases  in  size  and  thickens  upon  its  free  margin,  which 
reaches  ahnost  to  the  upper  border  of  the  short  atrial  canal  (Fig.  142) 
The  continuity  of  the  two  atria  is  thus  almost  dissolved,  but  is  soon 
re-estabhshed  by  the  formation  in  the  dorsal  part  of  the  septum  of 
an  opening  which  soon  reaches  a  considerable  size  and  is  known  ac 


Fig.  140.— Innie  Suiface  of  nre  Heabi  of  an  Embryo  of  10  lui 
■  *'"'^''"W™'«  tWa^lng;  sp.  septum  spuriu^n;  ,.  Wptum  primum;  sv.  „p,ua, 
ventncuh;  ve,  Eustachian  valve.— (i?ii.)  .      ■      i     «■ 

the  foramen  ovale  (Fig.  14,,  ».  Close  to  the  atrial  septum,  and 
parallel  with  it,  a  second  ridge  appears  in  the  roof  and  ventral  waU 
of  the  right  atrium.     This  septum  secundum  (Fi,  ,    S  )  is  of 

relatively  slight  development  in  the  human  embi'>.  and  its  free 
edge,  arching  around  the  ventral  edge  and  floor  of  the  foramen 
ovale,  becomes  continuous  with  the  left  lip  of  the  fold  which  guards 
the  opemng  of  the  sinus  vcnosus  and  with  this  forms  the  otmuius 
of  Vieussens  of  the  adult  heart. 


2,34 


THE   rOMIAnON   OF   TEE  HEAKT 


When  the  absorption  of  the  sinus  venosus  into  the  wall  of  the 
right  atritun  has  proceeded  so  far  that  the  veins  communicate 
directly  with  the  atrium,  the  vena  cava  superior  opens  into  it  at  the 
upper  part  of  the  dorsal  wall,  the  vena  cava  inferior  more  laterally, 
and  below  this*is  the  smaller  opening  of  the  coronary  sinus.  The 
upper  portion  of  the  right  lip  of  the  lold 
which  originally  surrounded  the  opening 
of  the  sinus  venosus,  together  with  the 
septum  spurium,  gradually  disappears; 
the  lower  portion  persists,  however,  and 
forms  (i)  the  Eustachian  valve  (Fig.  141, 
Ve),  guarding  the  opening  of  the  inferior 
cava  and  directing  the  blood  entering  by 
it  toward  thie  foramen  ovale,  and  (2)  the 
Thebesian  valve,  which  guards  the  open- 
ing of  the  coronary  sinus.  At  first  no 
Fio.  141.— Heart  01  Ehbryo   veins  commimicate  with  the  left  atrium, 

or  lO.J  CM.  I»OK  WHICH  HalJ     .      ,  .t       J  1  .     r  .t      1  j 

or  THE  Right  Aueicli  ttvs   but  on  the  development  of  the  lungs  and 
BEEN  Removed.  ^g  establishment  of  their  vesseb,  the 

/tf,  Foramen  ovale:  fa,  pul-         ,  ,  ,  ,.  .., 

monary  anery;  s„  septum  pri-   pulmonary  vcms  make  connection  with 
mum;  s„  sepmm  secundum;   jj     ^wo  veins  arise  from  each  lung,  and 

oa.  systemic  aorta;  K,  right  ven-  '^ 

uide;  vci  and  ves,  inferior  and  as  they  pass  toward  the  heart  they  unite 
^m^Lve!"  "™'  ''  ""  '  in  pairs,  the  two  vesseb  so  formed  again 
uniting  to  form  a  single  short  trunk  which 
opens  into  the  upper  part  of  the  atrium  (Fig.  142,  Vep).  As  is  the 
case  with  the  right  atrium  and  the  sinus  venosus,  the  expansion  of 
the  left  atrium  brings  about  the  absorption  of  the  short  single  trunk 
into  its  walls,  and,  the  expansion  continuing,  the  two  vessels  are  also 
absorbed,  so  that  eventually  the  four  primary  veins  open  independ- 
ently into  the  atrium. 

While  the  atrial  septa  have  been  developing  there  has  appeared 
on  the  dorsal  wall  of  the  atrial  canal  a  tubercle-like  thickening  of 
the  endocardium,  and  a  similar  thickening  also  forms  on  the  ventral 
wall.  These  endocardial  ctishions  increase  in  size  and  finally  unite 
together  by  their  tips,  forming  a  complete  partition,  dividing  the 


THE   rOSMATION  OP  THE  HEAST 


'35 


atrial  canal  into  a  right  and  left  half  (Fig.  14a).  With  the  upper 
edge  of  this  partition  the  thickened  lower  edge  of  the  atrial  septum 
unites,  so  that  the  separation  of  the  atria  would  be  complete  were  it 
not  for  the  foramen  ovale. 


En.s 


FlO.    141.— SlCnOH    IHSOUOH   A    RlCONSTKUCTION    Or    THE    HeAM    OF   A    RASBIT 
EhbKYO  of  10. 1  HH. 

,h\^h  ^A^il:  ^^h*  T'^.u^'-J'"  "?■'""!  *"■■  »"<'  ^'-  '<"«'  »<b  Of  the  ridges 

nrimf™^   S'/.  ■  "'.'°J"^i  '^  '":  ■?""«'ri':''lar  communication;  S„  septuS 
?w  iSj  '  ^^l  "'•''  •^^'  ''"  •""?  "'  •*'  »'■""  «"<»'»;  -S'".  ventricuUi  septum; 

«yJSS^„I^'  ^'^-.P"'-?"?"):  '"";  »'■"'  and  Vvs,  iTght  and  left  Umbs  of  the 
valve  guarding  the  openmg  of  the  sinus  venosus.— (Born.) 

While  these  changes  have  been  taking  place  in  the  atrial  portion 
of  the  heart,  the  separation  of  the  right  and  left  ventricles  has  also 
been  progressing,  and  in  this  two  distinct  septa  take  part.  From 
the  floor  of  the  ventricular  cavity  along  the  line  of  junction  of  the 


836 


THE  FORMAnOM  Of  TBZ  HEAKT 


right  and  left  portions  a  ridge,  composed  laigely  of  muscular  tissue, 
arises  (Figs.  140  and  143),  and,  growing  more  rapidly  in  its  dorsal 
than  its  ventnl  portion,  it  comes  into  contact  and  fuses  with  the 
dorsal  part  of  the  partition  of  the  atrial  canal.  Ventrally,  however, 
the  ridge,  known  as  the  ventricular  septum,  fails  to  reach  the  ventral 
part  of  the  partition,  so  that  an  oval  foramen,  situated  just  below  the 
point  where  the  aortic  bulb  arises,  still  remains  between  the  two 
ventricles.  This  opening  is  finally  closed  by  what  it  termed  the 
aorlic  septum.  This  makes  its  appearanc  in  the  aortic  bulb  just  at 
the  point  where  the  first  lateral  branches  which  give  origin  to  the 
pulmonary  arteries  (see  p.  243)  arise,  and  is  formed  by  the  fusion 
of  the  free  edges  of  two  endocardial  ridges  which  develop  on  opposite 
sides  of  the  bulb.  From  its  point  of  origin  it  gradually  extends 
down  the  bulb  until  it  reaches  the  ventricle,  where  it  fuses  with 
the  &ee  edge  of  the  ventricular  septum  and  so  completes  the  separa- 
tion of  the  two  ventricles  (Fig.  143).  The  bulb  now  consists  of  two 
vessels  lying  side  by  side,  and  owing  to  the  position  of  the  partition 
at  its  anterior  end,  one  of  these  vessels,  that  which  opens  into  the 
right  ventricle,  is  continuous  with  the  pulmonary  arteries,  while  the 
other,  which  opens  into  the  left  ventricle,  is  continuous  with  the  rest 
of  the  vessels  which  arise  from  the  forward  continuation  of  the  bulb. 
As  soon  as  the  development  of  the  partition  is  completed,  two  grooves, 
corresponding  in  position  to  the  lines  of  attachment  of  the  partition 
on  the  inside  of  the  bulb,  make  their  appearance  on  the  outside  and 
gradually  deepen  until  they  finally  meet  and  divide  the  bulb  into  two 
separate  vessels,  one  of  which  is  the  pulmonary  aorta  and  the  other 
the  systemic  aorta. 

In  the  early  stages  of  the  heart's  development  the  muscle  bundles 
which  compose  the  wall  of  the  ventricle  are  very  loosely  arranged, 
so  that  the  ventricle  is  a  somewhat  spongy  mass  of  muscular  tissue 
with  a  relatively  small  cavity.  As  development  proceeds  the  bundles 
nearest  the  outer  surface  come  closer  together  and  form  a  compact 
layer,  those  on  the  inner  surface,  however,  retaining  their  loose 
arrangement  for  a  longer  time  (Fig.  143).  The  lower  edge  of  the 
atrial  canal  becomes  prolonged  on  the  left  side  into  one,  and  on  the 


iBi  FouunoN  or  ihk  heait 


»31 


right  side  into  two,  flaps  wUch  project  downward  into  the  ventricular 
cavity,  and  an  additional  flap  arises  on  each  side  from  the  lower 


Xki:tl- 


S.m 


^  of  the  partition  of  the  atrial  canal,  so  that  three  flaps  occur  in 
tne  right  atrioventricular  opening  and  two  in  the  left.    To  the 


'3» 


THX  rouunoM  or  irk  heaxt 


under  aurfaces  of  these  flaps  the  loosely  arranged  muscular  tra- 
becube  of  the  ventricle  are  attached,  and  musctilar  tissue  also  occurs 
in  the  flaps.  This  condition  is  transitory,  however;  the  muscular 
tissue  of  the  flaps  degenerates  to  form  a  dense  layer  of  connective 
tissue,  and  at  the  same  time  the  muscular  trabeculte  undergo  a 
condensation.  Some  of  them  separate  from  the  flaps,  which  repre- 
sent the  atrio-veniricular  vahes,  and  form  muscle  bundles  which 
may  fuse  throughout  their  entire  length  with  the  more  compact 
portions  of  the  ventricular  walls,  or  else  may  be  attached  only  by 
their  ends,  forming  loops;  these  two  varieties  of  muscle  bundles 
constitute  the  irabecula  comae  of  the  adult  heart.    Other  bundles 


Fia.  144.— DuGRAHS  sBowiHO  1H>  DivxtonoMT  OF  TBI  Atnicui;o-vsMnict)Lut 

Valvzs. 

ft.  Muscular  trabeculie;  cht,  chordc  tendine;  mk  and  mk^,  valve; ^,  rausculus  papiUaris; 

le,  trabecule  cameie;  v,  ventricle. — (FroM  Htrtwig,  itfltr  Cegaiaur^ 


may  retain  a  transverse  direction,  passing  across  the  ventricular 
cavity  and  forming  the  so-called  moderator  bands;  while  others,  again, 
retaining  their  attachment  to  the  valves,  condense  only  at  their  lower 
ends  to  form  the  musculi  papiUares,  their  upper  portions  under- 
going conversion  into  strong  though  slender  fibrous  cords,  the 
chorda  lendinea  (Fig.  144). 

The  endocardial  lining  of  the  ventricles  is  at  first  a  simple  sac 
separated  by  a  distinct  interval  from  the  myocardium,  but  when  the 
condensation  of  the  muscle  trabecule  occurs  the  endocardium  applies 
itself  closely  to  the. irregular  surface  so  formed,  dipping  into  all  the 
crevices  between  the  trabecule  cameee  and  wrapping  itself  around 


TH«  rOUCATION  OF  THE  BEUT 


«39 


FlO.    145— DUGIAHS 
lUCSTIATDIO  THE  FoX- 

lUTioN  or  TBI  Sim- 
LONA«  Via.vn.-(Getm- 
Aow.) 


the  musculi  papillares  and  chord,  tendinee  so  as  to  form  a  complete 

limng  of  the  imier  surface  of  the  myocardium. 

The  aortic  and  pulmonary  wwi/imafra/rM  make  theirappearance, 

^Z",   ,  ..*°"''  ''""'  undeiKoes  its  longitudinal  splitting,  as  four 

tubercle-Uke  thickenings  of  connective  tissue  situated  on  the  inner 

wall  of  the  bulb  just  where  it  arises  from  the  ventricle.     When  the 

division  of  the  bulb  occurs,  two  of  the  thickenings,  situated  on 

opposite  sides,  are  divided,  so  that  both  the 

puhnonary  and  systemic  aorte  receive  three 

thickenings  (Fip.  145)-    Later  the  thickenings 

become  hollowed  out  on  the  surfaces  directed 

away  from  the  ventricles  and  are  so  converted 

into  the  pouch-like  valves  of  the  adult. 

Changes  in  Ae  Hearl  after  BirA.— The 
heart  when  first  formed  lies  far  forward  in  the 
neck  region  of  the  embryo,  between  the  head 
and  the  anterior  surface  of  the  yolk-sac,  and 
from  this  position  it  gradually  recedes  until  it  reaches  its  final 
position  in  the  thorax.  And  not  only  does  it  thus  change  its  rela- 
tive position,  but  the  direction  of  its  axes  ah»  changes  For  at  an 
early  stage  the  ventricles  lie  directly  in  front  of  {i.  ,.,  ventrad  to) 
the  atna  and  not  below  them  as  in  the  adult  heart,  and  this  prim- 
itive condition  is  retained  until  the  diaphragm  has  reached  its  final 
position  (see  p.  322). 

In  addition  to  these  changes  in  position,  which  are  antenatal 
important  changes  also  occur  in  the  atrial  septum  after  birth' 
Throughout  the  entire  period  of  fetal  life  the  foramen  ovale  persists 
permitting  the  blood  returning  from  the  placenta  and  entering  the 
right  atrium  to  pass  directly  across  to  the  left  atrium,  thence  to  the 
left  ventricle,  and  so  out  to  the  body  through  the  systemic  aorta 
(see  p.  267).  At  birth  the  lungs  begin  to  function  and  the  placental 
circulation  is  cut  o«f,  so  that  the  right  airium  receives  only  venous 
blood  and  the  left  only  arterial;  a  persisteiice  of  the  foramen  ovale 
beyond  this  period  would  be  injuriou5,  since  it  would  permit  of  a 
njiiture  of  the  arterial  and  venous  bloods,  and,  consequently   it 


340 


OEVZLOmZKT  or  THX  ABTXIML  SySTEII 


cloies  completely  soon  after  birth.  The  cloiure  ii  nuuie  poMible 
by  the  fact  that  during  the  growth  of  the  heart  in  size  the  portion  of 
the  atrial  teptum  which  is  between  the  edge  of  the  foramen  ovale 
and  the  dorsal  wall  of  the  atrium  increases  in  width,  so  that  the  fora- 
men is  carried  ftirther  and  further  away  from  the  dorsal  wall  of  the 
atrium  and  comes  to  be  almost  completely  overlapped  by  the  annulus 
o>'  Vieussens  (Fig.  141).  This  process  continuing,  the  dorsal  portion 
of  the  atrial  septum  finally  overlaps  the  free  edge  of  the  annulus, 
and  after  birth  the  fusion  of  the  overlapping  surfaces  takes  place  and 
the  foramen  is  completely  closed. 

In  a  large  percentage  (15  to  30  per  cent.)  of  individuals  the  fusion  of 
the  surfaces  of  the  septum  and  annulus  is  not  complete,  so  that  a  slit-like 
opening  persists  between  the  two  atria.  This,  however,  does  not  allow  of 
any  mingling  of  the  blood  in  the  two  cavities,  since  when  the  atria  contract 
the  pressure  of  the  blood  on  both  sides  will  force  the  overlapping  folds 
together  and  so  practically  close  the  opening.  Occasionally  the  growth 
of  the  dorsal  portion  of  the  septum  is  imperfect  or  is  inhibited,  in  which 
case  closure  of  the  foramen  ovale  is  impossible. 

The  DeTelopment  of  the  Arterial  System.— It  has  been  seen 
(p.  aai)  that  the  formation  of  the  blood-vessels  begins  in  the  extra- 
embryonic splanchnic  mesoderm  surrounding  the  yolk-sac  and  ex- 
tends thence  toward  the  embryo.  Furthermore,  it  has  been  seen 
thaf  the  vessels  appear  as  capillary  networks  from  which  definite 
stems  are  kter  elaborated.  This  seems  also  to  be  the  method  of 
formation  of  the  vessels  developed  within  the  body  of  the  embryo, 
the  arterial  and  venous  stems  being  first  represented  by  a  number 
of  anastomosing  capillaries,  from  which,  by  the  enlargement  of  some 
and  the  disappearance  of  the  others,  the  definite  stems  are  formed. 

The  earliest  known  embryo  that  shows  a  blood  circulation  is 
that  described  by  Etemod  (Fig.  43).  From  the  plexus  of  vessels 
on  the  yolk-sack  two  veins  arise  which  unite  with  two  other  veins 
returning  from  the  chorion  by  the  belly-stalk  and  passing  forward  to 
the  heart  as  the  two  umbilical  veins  (Fig.  146,  Vu).  There  is  as  yet 
no  vitelline  vein,  the  chorionic  circulation  in  the  human  embryo 
apparently  taking  precedence  over  the  vitelline.  From  the  heart 
a  short  arterial  stem  arises,  which  soon  divides  so  as  to  form  three 


MVEIOPMENT  or  THE  A.TMUt  SYITEM  ,„ 

rf^*)  which  extend  ainnTJZ^^  '^JT'  '°""'  """^  C^'' 
yet  no  sign  of  vitelline  arf,w~  „  "™nes  (^u).  There  is  as 
.n  indication  of  th^t^C-re^Tvili::  .rr^t'  '«"» 
circulation  in  the  human  embryo.  ""^  '''°"°"'<= 


-rcn  vessels,  and  later  also  ihe  two  dorsal 


f 


2^3  DBVEtOPiaKr  OF  IHE  AKTEUAL  SYSTEM 

aorte  fuse  as  far  forward  as  the  region  of  the  eighth  cervical  segment 
to  form  a  single  trunk  from  which  segmental  branches  arise. 

It  wiU  be  convenient  to  consider  first  the  history  of  the  vessels 
which  pass  dorsaUy  in  the  branchial  arches.    Altogether,  six  cf  these 
vessels  are  developed,  the  fifth  being  rudimentary  and  transitory,  and 
when  fully  formed  they  have  an  arrangement  which  may  be  under- 
stood from  the  diagram   (Fig. 
147).    This  arrangement  repre- 
sents a  condition  which  is  per- 
manent in  the  lower  vertebrates. 
In  the  fishes  the  respiration  is 
performed   by   means    of   gills 
developed  upon  the    branchial 
arches,  and  the  heart  is  an  organ 
which  receives  venous  blood  from 
the  body  and  pimips  it  to  the 
giUs,  in  which  it  becomes  arte- 
rialized  and  is  then  collected  into 
the  dorsal  aortse,  which  distrib- 
ute it  to  the  body.    But  in  terres- 
trial animals,  with  the  loss  of  the 
gills  and  the  development  of  the 
hmgs  as  respiratory  organs,  the 
capillaries  of  the  gills  disappear 
and   the   afferent  and  efferent 
branchial  vessels  become  con- 
tinuous,   the    condition   repre- 
sented in  the  diagram  resulting. 
But  this  condition  is  merely  temporary  in  the  mammalia  and 
numerous  changes  occur  in  the  arrangement  of  the  vessels  before 
the  adult  plan  is  realized.    The  first  change  is  a  disappearance  of 
the  vessel  of  the  first  arch,  the  ventral  stem  from  which  it  arose  being 
continued  forward  to  form  the  temporal  arteries,  giving  off  near  the 
point  where  the  branchial  vessel  originaUy  arose  a  branch  which 
represents  the  internal  maiillary  artery  in  part,  and  possibly  also  a 


FlO.  147.— DlAO»AM  Illdst»atino  th« 
FxnuxT  AiMsoaam  o»  the  Bran- 
chial AkCB  VlSSXLS. 

0,  aorta;  ab,  aortic  bulb;  te,  external 
carotid;  ic,  internal  carotid;  «,  subclavian; 
I-VI,  branchial  arch  nssds. 


DEVBLOPMNT  OF  IHE  ABTEWAl  8VSWM  ,43 


branchial  vessels.— (H»i.)  '  P'™«tenl 

ti™  wV  ?'  f^"*  ^T''  ^""""^  "•«  ^^«""''  ^"""'-d.  and  the  por- 
tion which  intervenes  between  the  third  and  fourth  y^sseh^oml 
the  common  carotid  (Fig.  149).  oecomes 

but  TJr  "«'''"»tary  fifth  vessel,  like  the  first  and  second,  disappear, 
bm  the  fourth  persists  to  form  the  aortic  arch,  there  b;ing  aftMs 
stage   of  development   two   complete   aortic   archw     f1™   .7. 
s^h  ve     1  .  branch  arises  wMch^passes "^1.::^^  SiMun^' 

Si  T'h"°r  ""'1'  '""  *^  P°"'°°  "f  *e  vessel  Se' 
r^nl  'T.'^'^h  mtervenes  betwern  this  and  the  aortic  arch  dis' 
appear,,  while  the  corresponding  portion  of  the  left  side^tL 


»44 


DEVELOFKENT  OF  THE  ARTEUAL  SYSTEM 


!  'ft 


until  after  birth,  forming  the  ductus  arteriosus  (ductus  Botalli)  (Fig. 
149).  When  the  longitudinal  division  of  the  aortic  bulb  occurs 
(p.  236),  the  septum  is  so  arranged  as  to  place  the  sixth  arch  in 
communication  with  the  right  ventricle  and  the  remaining  vessels 
in  connection  with  the  left  ventricle,  the  only  direct  communication 

between  the  systemic  and 
pulmonary  vessels  being  by 
way  of  the  ductus  arteriosus, 
whose  significance  will  be  ex- 
plained later  (p.  367). 

One  other  change  is  still 
necessary  before  the  vessels 
Acquire  the  arrangement 
which  they  possess  during 
fetal  life,  and  this  consists  in 
the  disappearance  of  the 
lower  portion  of  the  right 
aortic  arch  (Fig.  149),  so  that 
the  left  arch  alone  forms  the 
connection  between  the  heart 
and  the  dorsal  aorta.  The 
upper  part  of  the  right  aortic 
arch  persists  to  form  the  prox- 
imal part  of  the  right  sub- 
clavian artery,  the  portion  of 
the  ventral  trunk  which  unites 
the  arch  with  the  aortic  bulb 
becoming  the  innominate 
artery. 

From  die  entire  length  of  the  thoracic  aorta,  and  in  the  embryo 
from  the  aortic  arches,  lateral  branches  arise  corresponding  to  each 
segment  and  accompanying  the  segmental  nerves.  The  first  of 
these  branches  arises  just  below  the  point  of  union  of  the  vessel 
of  the  sixth  arch  with  the  dorsal  trunk  and  accompanies  the  hypo- 
glossal nerve  (Fig.  150,  *),  and  that  which  accompanies  the  seventh 


FlO.  149. — DlAOIAK  Illotteatwo  thi 
CBAHOES  IN  IBS  BKAHCHXAL  AkCB  VESSELS. 

a,  AorU;  ia,  ductus  arteriosus;  tc,  eztemal 
carotid;  ic,  intemal  carotid;  pa,  pulmonary  ar- 
tery; sc,  subclavian;  /-K/,  aortic  arch  vessels. 


DEVKIOPMOT  0»  IHB  ARTERIAL  SMTEH 


»4S 


>  aortic 


HL&^^i-  ""*  ^'^'^  -""  ■•"•»  «•''  «•»•>  bud,  fonning  the 


resent  the  intercostal  arteries, 
and  still  lower  four  pairs  of  lum- 
bar arteries  are  formed,  the  fifth 
lumbars   being  represented  by 
two  large  branches,  the  common 
iliacs,  which  seem  from  their  size 
to  be  the  continuations  of  the 
aorta  rather  than  branches  of  it. 
The    true  continuation   of  the 
aorta  is,  however,  the  middle  sa- 
cral artery,  which  represents  in 
a  degenerated  form  the  caudal 
prolongation  of    the    aorta   of 
other  mammals,  and,  like  this, 
gives  off  lateral  branches  corre- 
sponding to  the  sacral  segments. 
In  addition  to  the  segmental 
lateral    branches   arising   from 
the  aorta,   visceral    branches, 


"3. 

DC 


^^«     /Af 


Flo.  »SO.— DlAOKAM  9&OWINO  m  p. 

passing  to  the  mesonephros  (see  d    «o^  r^l     ^  ^** 

the  Daired   h«„-k  •         '^^  ^•^''  corresponding  to  each  of 

tne  paired  branches  passing  to  the  body  waU  (Fig.   15,).    As 


346  DEVXIOPHENT  01  THE  ARTEIUL  SYSTEM 

development  proceeds  the  great  majority  of  these  visceral 
branches  disappear,  certain  of  the  lateral  ones  persisting,  however, 
to  form  the  renal,  internal  spermatic,  and  hypogastric  arteries  of 
the  adult,  while  the  unpaired  branches  are  represented  only  by  the 
cceliac  artery  and  the  superior  and  inferior  mesenteries.  The 
superior  mesenteric  artery  is  the  adult  representative  of  the  vitelline 
artery  of  the  embryo  and  arises  from  the  aorta  by  two,  three  or  more 
roots,  which  correspond  to  the  fifth,  fourth  and  higher  thoracic 


FlO.    151. — DiAOKAlC  SHOWING  THK   AUtANOXMENT   OF  THE   SEGHXNTAL   BSAMCmS 
AKISINO  rXOK  THS  AOHTA. 

A,  Aorta;  B,  lateral  aomatic  branch;  e,  lateral  visceral  branch;  D,  median  viiceral 
branch;  £,  peritoneum. 

segments.  Later,  all  but  the  lowest  of  the  roots  disappear  and  the 
persisting  one  undergoes  1  downward  migration  in  accordance  with 
the  recession  of  the  diaphragm  and  viscera  (see  p.  322),  imtil  in 
embryos  of  17  nun.  it  lies  opposite  the  first  lumbar  segment.  Simi- 
larly the  cceliac  and  inferior  mesenteric  arteries,  which  when  first 
recognizable  in  embryos  of  9  mm.  correspond  with  the  fourth  and 
twelfth  thoracic  segments  respectively,  also  undergo  a  secondary 
downward  migration,  the  cceliac  artery  in  embryos  of  17  mm.  arising 


DEVELOPMENT   OP  THE  A«TE«ML  SYSTEM  247 

nf^-!!  ?^  r*"*  '^""^^  "''  *^'  '°f'="°'  "esenteric  opposite 
the  third  lumbar  segment. 

The  umbilical  arteries  of  the  embryo  seem  at  first  to  be  the  direct 
contmuations  of  the  dorsal  aorUe  (Fig.  146).  but  as  development 
proceeds  they  come  to  arise  from  the  aorta  opposite  the  third 
lumbar  segment  where  they  are  in  line  with  the  lateral  visceral 
segmental  branches.    They  pass  ventral  to  the  Wolffian  duct  (see 
P-  339)  and  are  continued  out 
ulong  with  the  allantois  to  the 
chorionic    villi.     Later    this 
original  stem  is  joined,  not  far 
from  its  origin,  by  what  ap- 
pears to  be  the  lateral  somatic 
branch  of  the  fifth  lumbar  seg- 
ment, whereupon  the  proximal 
part  of  the  original  umbilical 
vessel  degenerates  and  the  um- 
•  bilical  comes  to  arise  from  the 
somatic  branch,  which  is  the 
common  iliac  artery  of  adult 
anatomy   (Fig.   152).    Hence 
it  is  that  this  vessel  in  the  adult 
gives  origin  both  to  branches 
such  as  the  external  iliac,  the 
gluteal,  the  sciatic  and  the  in- 
ternal   pudendal,   which   are 
distributed  to  the  body  walls 
or  their  derivatives,  and  to  others,  such  as  the  vesical,  inferior  hemor- 
rhoidal and  uterine,  which  are  distributed  to  the  pelvic  viscera.    At 
hinh  the  portions  of  the  umbilical  arteries  beyond  the  umbiUcus  are 
severed  when  the  umbilical  cord  is  cut,  and  their  intra-embryonic 
portions,  which  have  been  caUed  the  hypogastric  arteries,  quickly 
undergo  a  reduction  in  size.    Their  proximal  portions  remain 
lunctional  as  the  superior  vesical  arteries,  carrying  blood  to  the 
urinary  bladder,  but  the  portions  which  intervene  between  the 


Fro.  15J.— DUOMM  ILLDSTMTINO  TBS 
DSVELOPIONT  OF  THI  UmiLrcAL  A«TMtta. 

A,  Aorta;  CIl,  common  iliac;  Ell,  exter- 
nal Uiac;  G.  gluteal;  III,  internal  iliac;  IP 
mteraal  pudic;  /K  interior  vesical;  5c,  scia^ 
he;  U,  umbilical;  U,  primary  proximal  por- 
tion of  the  umbilical;  wd,  Wolffian  duct 


l!l^ 


§ 


348 


DEVELOPMENT  OF  THE  ABTESIAL  SYSTEM 


bladder  and  the  umbilicus  become  reduced  to  solid  cords,  forming 
the  obliterated  hypogastric  arteries  of  adult  anatomy. 

In  its  general  plan,  accordingly,  the  arterial  system  may  be 
regarded  as  consisting  of  a  pair  of  longitudinal  vessels  which  fuse 
together  throughout  the  greater  portion  of  their  length  to  form 
the  dorsal  aorta,  from  which  there  arise  segmentally  arranged 
lateral  somatic  branches  and  ventral  and  lateral  visceral  branches. 
With  the  exception  of  the  aortic  tnmks  (together  with  their  anterior 
continuations,  the  internal  carotids)  and  the  external  carotids,  no 
longitudinal  arteries  exist  primarily.  In  the  adult,  however,  several 
longitudinal  vessels,  such  as  the  vertebrals,  internal  mammary, 
and  epigastric  arteries,  exist.  The  formation  of  these  secondary 
longitudinal  trunks  is  the  result  of  a  development  between  adjacent 
vessels  of  anastomoses,  which  become  larger  and  more  important 
blood-channels  than  the  original  vessels. 

At  an  early  stage  each  of  the  lateral  branches  ot  the  dorsal  aorta 
gives  off  a  twig  which  passes  forward  to  anastomose  with  a  back- 
wardly  directed  twig  from  the  next  anterior  lateral  branch,  so  as  to 
form  a  longitudinal  chain  of  anastomoses  along  each  side  of  the 
neck.  In  the  earliest  stage  at  present  known  the  chain  starts  from 
the  lateral  branch  corresponding  to  the  first  cervical  (suboccipital) 
segment  and  extends  forward  into  the  skull  through  the  foramen 
magnum,  terminating  by  anastomosing  with  the  internal  carotid. 
To  this  original  chain  other  links  are  added  from  each  of  the 
succeeding  cervical  lateral  branches  as  far  back  as  the  seventh 
(Figs.  150  and  133).  But  in  the  meantime  the  recession  of  the 
heart  toward  the  thorax  has  begim,  with  the  result  that  the  common 
carotid  stems  are  elongated  and  the  aortic  arches  are  apparently 
shortened  so  that  the  subclavian  arises  on  the  left  side  almost 
opposite  the  point  where  the  aorta  was  joined  by  the  sixth  branchial 
vessel.  As  this  apparent  shortening  proceeds,  the  various  lateral 
branches  which  give  rise  to  the  chain  of  anastomoses,  with  the 
exception  of  the  seventh,  disappear  in  their  proximal  portions  and 
the  chain  becomes  an  independent  stem,  the  vertebral  artery,  arising 
from  the  seventh  lateral  branch,  which  is  the  subclavian. 


DEVXtOPMENT  OF   THE  ABTEMAt  SYSTEM  J45 

The  recession  of  the  heart  is  continued  until  it  lies  below  th,. 
oil  "\Ty  '"'^"°^'^'  ''"'="'=^'  -<»  'he  upp^r  two  oHhi 
rwth'Ii  H  r  "■"■"'  '""^^  "^  '^^h  ^"»«=  -^^  'heir  onnt' 
t.on  w,th  the  dorsal  aorta,  and,  sending  off  anteriorly  and  posteriorly 


-Av.<z{ 


-..As 


a.d  C.  inLna,  and  -..rni'cSr^cXS^XC^li^^^S^j^'^^  C" 

anastomosing  twigs,  develop  a  short  longitudinal  stem,  the  superior 
.n/«-co./a/,  which  opens  into  the  subclavian.  ^ 

The    intercostals    and    their    abdominal    representatives,    the 


i 


T 


as© 


SEVEiAPimre  or  astkues  or  uus 


tumbars  and  iliacs,  alio  give  rise  to  longitudinal  anastomosing 
twigs  near  their  ventral  ends  (Fig.  154),  and  these  increasing  in 
size  give  rise  to  the  tntemal  mammary  and  mf trior  epigastric  arteries, 
which  together  form  continuous  stems  extending  from  the  sub- 
clavians  to  the  external  iliacs  in  the  ventral  abdominal  walls.  The 
suferficiai  epigaslrics  and  other  secondary  longitudinal  vessels  are 
formed  in  a  similar  manner. 

The  DeTelopment  of  the  Arteries  of  the  Limbs.— The  earliest 
stages  in  the  development  of  the  limb  arteries  are  unknown  in  man. 


Fio.  154.— Embeyo  oy  13  jm.  showino  thx  Mora  of  Development  or  ibi  Internal 
Maiomiiy  and  Deep  Epioastuc  Axtexies.— (MoM.) 


but  it  has  been  found  that  in  the  mouse  .lie  primary  supply  of  the 
anterior  limb  bud  is  from  five  branches  arising  from  the  sides  of  the 
aorta.  These  anastomose  to  form  a  plexus  from  which  later  a  single 
stem,  the  subclavian  artery,  is  elaborated,  occupying  the  position 
of  the  seventh  cervical  segmental  vessel,  the  remaining  branches  of 
the  plexus  having  disappeared.    The  common  iliac  artery  similarly 


mnionoHT  of  amkues  o»  umu  951 

also  M  eUborated  from  8  plexus  is  as  yet  unknown 

kn^'^a^"  """'^  °^  .*"  ""•'  '"*"■"  "  »'«'  but  imperfectly 

«atomy  and  on  the  anomalies  that  occur  in  the  adult  for  indications 

."naSL^t  *""*"'  "  "''"'''  *"  ^-    ^'  comparative  evidence 

.^.cate.  the  existence  of  several  stages  in  the  development  of  the 

!™fi™T  '  "f  "'  '"  •"  embryological  observations  go  they 
conton  the  conclusions  drawn  from  this  somxe,  although  the  vario  J 
Stages  show  apparently  a  great  amount  of  overlapping  owing  to  a 
Zf  r"°Lf  ''*=  «»-«'°P»-tal  stages.    In  the  sL^t  S^e 

for  lit  V    '  "^'"'  "^'^ ''  '*''^'*« '"'°  digital  branches 

for  the  fingers.    In  its  course  through  the  forearm  it  Ues  in  the 

mlTJ.  T  ''k-  """'"^  '^'^  "'^'  ^'■■»«  "^  'he  interosseous 
'^ssea  In  the  second  stage  a  new  artery  accompanying  the 
"tv  T,"  *Pp»«'  "i^''«  from  the  main  stem  or  brachial 
artery  a  httle  below  the  elbow-joint.  This  may  be  termed  the 
^  med^na  and  as  it  develops  the  arteria  intero^ea  gradually 
dmumshes  m  s.ze,  becoming  finaUy  the  small  volar  intm,sseous 
artery  of  the  adult  (Fig.  ,5s),  and  the  median,  uniting  ~ts 

cipal  stem  of  the  forearm.  '^ 

fmm  .t't  "T  ,''  'k'^u  "f^""^  '"  ''^  ""=  appearance  of  a  branch 
fa.m  the  brachial  wh,ch  forms  the  arleria  ulnaris.  and  this,  passing 
down  the  ulnar  side  of  the  forearm,  unites  at  the  wnst  Z.  thf 
median  to  form  a  superficial  palmar  arch  from  which  the  digital 
tamches  anse.  A  fourth  stage  is  marked  by  the  diminution  of  the 
median  arteiy  unt.1  it  finally  appears  to  be  a  small  branch  of  the 
interosseous,  and  at  the  same  time  there  develops  from  the  brachial, 
atabout  the  middle  of  the  upper  arm,  what  is  known  as  the  arJZ 

sule  of  the  forearm,  following  the  course  of  the  radial  nerve,  and  at 
the  wnst  passes  upon  the  dorsal  surface  of  the  hand  to  form  the 


V 


*s* 


mvELonuMT  or  akiuiu  or  urns 


dorsal  digital  arteries  of  the  thumb  and  index  finger.  At  fint  thia 
artery  takes  no  part  in  the  fomution  of  the  palmar  arches,  but  later 
it  gives  rise  to  the  superficial  volar  branch,  which  usually  unites 
with  the  superficial  arch,  while  from  its  dorsal  portion  a  perforating 
branch  develops  which  passes  between  the  first  and  second  meta- 


FlC.  155. — DUOIAHS  SHOWmO  an  EUILY  AND  A  LATI  STAOC  IN  IHI  DiVILOnailT 

or  THE  AxrxMoa  or  thi  Axk. 
6,  Brachial;  i,  interosseous;  M,  median;  r,  radial;  rs,  supoficial  radial;  «,  ulnar. 


carpal  bones  and  unites  with  a  deep  branch  of  the  ulnar  to  form  the 
deep  arch.  The  fifth  or  adult  stage  is  reached  by  the  development 
from  the  biachial  below  the  elbow  of  a  branch  (Tig.  155,  r)  which 
passes  downward  and  outward  to  unite  with  the  superficial  radial, 
whereupon  the  upper  portion  of  that  artery  degenerates  until  it  i^^ 


MWttOMIlMT  0»  Ainmi  OF  umi  jj^ 

oue  ot  tne  upper  limb,  it  is  necessary  to  rely  larjfelv  on  the  fact,  nt 
comparafve  anatomy  and  on  anomalies  whfch  Sur  in  tte  C„ 

W  Um^  tr'  ":'  '?'"'""'  •'^^^"'P™-'  "f  'he  arteries  ofTh" 
LTk^  JL^'  "•'''"'''  '^"  "^  """  'he  common  iliac  artery 
^1^."^"^  "^  "  •""=""  ^"«^h  °^  'he  dorsal  aorta,  and  in  fhe 

^^es^ff'Tbl"^  '^'''-' "'  ^uVatiynn^;£r:  s:: 

pves  off  a  branch  corresponding  to  the  anterior  tibial  (a/)  vhich 
P««»g  forward  to  the  e«ensor  surface  of  the  leg,  quickly  SeTS 
«  the  extensor  muscles.  The  main  artery  continues  do«n^r2t 
Ae  mterosseous  membrane,  and  some  distance  aboveT  anu" 
divides  mto  a  strong  anterior  and  a  weaker  posterior  branch  the 
raTe^nr  f  r^'rr  ^'^•^  '^  -tinu^down  tSe'n^ 
tSL,  ^- *      '"""  *"= ''"'"'  P'^  °f  the  anterior  tibial  and 

surface  o  the  foot,  is  lost  in  the  plantar  muscles.    A^this  stL  th^ 
-rternal  ihac  is  a  secondary  branch  of  the  common  iliac,  bl^ij 
poorly  developed  and  not  extending  as  far  as  the  kn^  * 

eou?,  ^t '~°°"!  "^'  '^  '^'™"'  '"""^  '"'^'y  '""«=ases  in  size  until  it 
equab  the  sciatic,  and  it  now  penetrates  the  adductor  magnus 

S  tt'lor^'n'  "  ''"'  "^  ^  ^'""«  •'"'"^h  M  which  accom! 
pames  the  long  saphenous  nerve  down  the  imjer  side  of  the  leg,  and, 


1 


»S4 


DSVBLOmXNT  OF  AKISUSI  OF  UMM 


ptMing  behind  the  intenul  nulleohu,  extendi  upon  the  pkntar 
turface  of  the  foot,  where  it  give*  rise  to  the  digitml  brancbe*.  From 
thii  arrangement  the  adult  condition  may  be  derived  by  the  con- 
tinued increase  in  size  of  the  external  iliac  and  its  continuation,  the 
femoral  (/),  accompanied  by  a  reduction  of  the  upper  portimt  of  the 
sciatic  and  its  separation  from  its  popliteal  portion  (J)  to  form  the 
inferior  ghiteal  artery  of  the  adult.    The  continuation  of  the  popU- 


n 


/Fi 


n 


pe 


k 


^ 


•SK     at 


t\ 


k 


pt 


Fia.  156. — DUOUHS  lUUSTtATINO  STAOU  IM  the  DlVnOnOHT  01  IHZ  AlTSUU 

or  THE  Lzo. 

at,  Anterior  tibial;  dp,  donaUt  pedis;/,  femonl;  p,  popliteal;  pt,  peroneil  pt,  posterior 

tibial;  s,  sciatic  (inferior  gluteal);  so,  saplienous. 

teal  down  the  leg  is  the  peroneal  artery  (ft)  and  the  upper  perforating 
branch  of  this  unites  with  the  lower  one  to  form  a  continuous  ante- 
rior tibial,  the  lower  connection  of  which  with  the  peroneal  persists 
in  part  as  the  anterior  peroneal  artery.  A  new  branch  arises  from 
the  upper  part  of  the  peroneal  and  passes  down  the  back  of  the  leg 


nvxLonam  o»  mc  nimvt  iwnu  ,55 

become,  much  reduced,  per,i.ti„g  „  .he  .uperficial  b«nch  of  t^ 
accompany  the  long  saphenous  nerve. 

The  DtTtlopmuit  of  the  Venoui  Swtem.-The  earlie,t  vein, 
to  develop  are  those  which  accompany  th'Tfirst-for^'at  . T 
umbihcals,  but  it  will  be  more  convenient  to  consider  fir,,  u.  ^Z 
wh^h  canry  the  blood  from  the  body  of  the  embryo  ;.,  I:  .  ."e 
heart.  These  make  their  appearance,  while  the  hear,  i-  .1"  in  ,  .. 
Ir^^l  "''"'",•  "  ""'  P"'"  °'  '""git-dinal  trunfc  ,  anU-rior 
^^^  ^rrfmo/  vnns.  into  which  latenil  bntn-  lus.  „„„  J 

rr^     -^        \°'"°"''-    ^'''  ^"''°  «'«»•»»  f°™<«<i  from  .he  Vvt 
a  tie  ..de  o  the  notochord  and  is  continued  on  the  under     n 
of  the  bram,  lying  medial  to  the  roote  of  the  cranial  nerv^  "  U.er 
sprouts  arising  from  the  vein  form  loops  around  the  ne^^ts^ld 

^Xr;'  '  ■'  'T  r^  "^  '•'^  °"«'-'  veinZ  dapper' 
50  that  the  vessel  now  Ues  lateral  to  the  nerve  roots,  e«ept  in  the  case 

1!L  *""'""'■  *'^'=  '^  °"«^'  ^^^  persists  to  fo^tl 
««««««  sff^.  From  the  vena  capUis  laUrMs  so  formT^h^ee 
vems,  an  anterior,  a  middle  and  a  posterior  cerebral,  Z^.o^e 

mto  the  antenor  end  of  the  cavernous  sinus,  the  middle  cerebral  into 
he  postenor  extremity  of  the  same  sinus  and  the  pos terio"  cer  S 
into  the  vena  cap,fs  lateraUs  behind  the  ear  vesicle^ig.  ic,)  The 
bnmchesoftheanteriorcerebralvei„e:rtendingovertheLebilhe^^ 
^ereunue  wuhtheirfellowsof  the  opposite  side  to  formalongitu- 
dmal  tn^nk  the  superior  sagiM  sinus.  lying  between  the  twoTere- 
bral  hemispheres.  At  first  this  sinus  drains  by  way  of  the  anterior 
n^L''  •""  ^"'n  ^'' '')' '""  ="  "^^  cerebral  hemispheres  in^ 
Xh'e  miS;  ^"7^"!.^'''^"''  ''"•'  "^^  connectiotH^ 
(^V-  158,  B  and  C),  each  of  these  becoming  in  turn  the  principal 


!'  il 


as6 


DEVXIOPUENT  OF  THE   VENOUS  SVSTEH 


drainage  of  the  sinus.  The  connections  which  join  the  veins  to  the 
sinus  become  the  proximal  portion  of  the  transverse  sinus,  the  poste- 
rior cerebral  vein  itself  becoming  the  distal  portion,  the  middle 
cerebral  vein  becomes  the  superior  petrosal  sinus,  while  the  anterior 
cerebral  vein  persists  as  the  middle  cerebral  vein  of  adult  anatomy 


<f«  vd  pcv 


Fio.  157. — Reconstruction  of  the  Head  or  A  Huuan  Kubryo  of  9  uic.  showing 
THE  Cerebral  Veins. 
acv,  Anterior  ceiebral  vein;   om,  auditory  vesicle;  cs,  cavernous  sinus;  fa,  facial 
nerve;  mcv,  middle  cerebral  vein;  pcv,  posterior  cerebral  vein;  tr,  trigeminal  nerve; 
vd,  lateral  cerebral  vein. — {Mall.) 


(Fig.  158,  C).  Additional  sprouts  from  the  terminal  portion  of  the 
superior  sagittal  sinus  give  rise  to  the  straight  and  inferior  sagittal 
sinuses,  and,  after  the  disappearacne  of  the  vena  capitis  lateralis,  a 
new  stem  develops  between  the  cavernous  and  transverse  sinuses, 
passing  medial  to  the  ear  vesicle,  and  forms  the  inferior  petrosal 
sinus  (Fig.  158,  C).    This  joins  the  transverse  sinus  at  the  jugular 


BKVBLOPMENT   OF  XHE   V^KOUS   SVS^M  ,57 

tnmk,  the  ductus  CmLlTiT''"^'""^'^^^i<i^>^common 
me<La„  hne  open  toS^des  oT"'""  "■'^"''^'^  '°-»««  *e 


segn^ental  veins  openfng™^^^^^  «"'=•  '^^  --ni 

recedes,  however,  the  f^'w"'"'  '^'"■'''""''-    ^s  the  heart 

^^  .         jugulars  become  more  and  more  elongated 


1 


ll 


»S8 


OEVKLOniBNT  01  THE   VEKOUS  SYSTXH 


and  the  cervical  lateral  veins  shift  their  communication  from  the 
cardinab  to  the  jiigulars,  imtil,  when  the  subclavians  have  thus 
shifted,  the  juguUtfs  become  much  larger  than  the  cardinals.  When 
the  sinus  venosus  is  absorbed  into  the  wall  of  the  right  auricle,  the 
course  of  the  left  Cuvierian  duct  becomes  a  little  longer  than  that 
of  the  right,  and  from  the  left  jugular,  at  the  ^loint  where  it  is  joined, 
by  the  left  subclavian,  a  branch  arises  which  extends  obliquely  across 
to  join  the  right  jugular,  forming  the  left  itmomittale  vein.  When 
this  is  established,  the  connection  between  the  left  jugular  and 
Cuvierian  duct  is  dissolved,  the  blood  from  the  left  side  of  the  head 
and  neck  and  from  the  left  subclavian  vein  passing  over  to  empty 


Fio,  159. — DuoxAHS  sBovnNG  THE  Devhapkint  or  THE  SupKUOs  Vbu  Cava 
0,  AzjrfOA  vein;  cs,  caronuy  sinus;  «j,  external  jugular;  k,  hepatic  vein;  ij,  inteinal 
jugular;  Mr  and  inl,  right  and  left  innominate  veins;  i,  subclavian;  vei  amd  ves,  inferior 
and  superior  venc  cavK. 

into  the  right  jugular,  whose  lower  end,  togethei  with  the  right 
Cuvierian  duct,  thus  becomes  the  superior  vena  cava.  The  left 
Cuvierian  duct  persists,  forming  with  the  left  horn  of  the  sinus 
venosus  the  coronary  sinus  (Fig.  159). 

The  external  jugular  vein  develops  somewhat  later  than  th> 
internal.  The  facial  vein,  which  primarily  forms  the  principil 
aflnent  of  this  stem,  passes  at  first  into  the  skull  along  with  the  fifi  1 
:  and  communicates  with  the  internal  jugular  system,  but  lat<'' 


™*   VSHOns  SYSTEM 


.  ^-    --""US  SYSTEM 

.  '^\»<='k  <«  the  external  iuX  T?  *'  J"'"  ""^  "'^nds  do,^ 
-th  the  ophthataic  at  ^„  ^r  f '  "><=  facial  anas,o«oZ 
connect  o„3  with  the  intenul  Sarts.  1/^'  "''  ""^  ^'«'  n^k« 
-xj  -  the  aduh  condition  is  a!:^^"^' '''"'  "  "^^  "ossed  the  Jaw 

dition  is  re^ln   K  f™!  J"*"'"  sy»«em     I„  ,f '"T***  ""hich  have 

The  posterior  cardinal  v^in. 
fermed,  the  cardinals,  extendTa^^^  "  '!"''  ""'  -"-^  -mply  be 

from  the  me^ntery  and  also  from  T  ""'  '^^^iving  ,eins 

vcms  of  the  neck  and  trunk  re^^s   11:T'  '"'''''  -~ 
the  Srst  cervical  segment  which  n^  ^*  ""^P"»n  of  that  of 

Jowev«-  as  alreadTSscrilS^t  T^^  ;""'  '^e  Jugular.     Lat/r 

cardinals  ^i^  ;„  ^,    ^P»  '^°  t^^"^'"*'-     f"  addition   fhe 

rsiE;  '•'''*  ^'^ «-  "-TClteT"-  ^■'^  ^ 

»t  wben  they  are  replaced  later  on  Zl  *""*"  embrro, 

(»etene^«,>  their  a«er,,nt  vdnlund    '      '  ^""'"'''^'  kid»^ 

cbaqgn  by  ^,e„  j,^  ""«  sue  of  the  cardinals.     The 

;;-a  that  their  de«^iorL  .'et^'^P™-'  "^  'he  inferior    I^^' 
h.«o;T  of  the  vitelTL,  -b"   a  v^^^^^^^^^  ^.U^ 

7  ^rroST^iThrrn^r" 

^'-^-sidesof^.J-^^ee.^^^- 


I 


|r 


26o 


nBvCLOFHSin  of  the  venous  sysnif 


sponding  umbilical  veins.  These  are  represented  in  the  belly- 
stalk  by  a  single  venous  trunk  which,  when  it  reaches  the  body  of 
the  embryo,  divides  into  two  stems  which  pass  forward,  one  on  each 
ude  of  the  umbilicus,  and  thence  on  each  side  of  the  median  line  of 
the  ventral  abdominal  wall,  to  form  with  the  corresponding  vitelline 
veins  common  trunks  which  open  into  the  ductus  Cuvieri.  As  the 
liver  develops  it  comes  into  intimate  relation  with  the  vitelline  veins, 
which  receive  numerous  branches  from  its  substance  and,  indeed, 
seem  to  break  up  into  a  network  (Fig.  i6o,  A)  traversing  the  liver 


MCr 


-DUOKAMS  IlXOSTBATINO  TH«  TKANSFOMIATIONS  Or  THE  VrTELLINI  ANIi 

UMBmcAi  Veins. 

DC,  Ductus  Cuvieri;  D.V.A,  ductus  venosus;  V.o.m.ii  and  V.o.m.s,  right  and  kfl 
vitelline  veins;  V.u.d  and  Vmj,  right  and  left  umbilical  veins.— (Hoc*s(<««'.) 

substance  and  uniting  again  to  form  two  stems  which  represent  the 
original  continuations  of  the  vitellines.  From  the  point  where  the 
common  trunk  formed  by  the  right  vitelline  and  umbilical  veins 
opens  into  the  Cuvierian  duct  a  new  vein  develops,  passing  down 
ward  and  to  the  left  to  unite  with  the  left  vitelline;  this  is  the  duclu^ 
venosus  {Fig.i6o,B, D.V.A).  In  the  meantime  three  cross-connei 
tions  have  developed  between  the  two  vitelline  veins,  two  of  whir', 
pass  ventral  and  the  other  dorsal  to  the  intestine,  so  that  the  latter  - 


(leveiopcd  between  each  .™k^-    i      -      ^'  ^^  "  connection  ie 
«elB.V^,.,sn,£„;^" .;"."!  "»  "™l««lln« 

;-c. ..  >»..^'™*™ror;,:f.xtrT'm" 

t-oviteUrnetoiKium^dTl,       ;'"""'"'• '"''"»'"" 

-^  ■^' -'» -  -  ■  ~-L' :rc"ESte";ifn': 


-=  *or  ■  ,:"nr-et :::  ^-ir-  - 

Whfle  th^  change.  We  U«^^Z^     '  T  ^^ 
vein,  „H^,U,  .^T,^^  ^^  tLTTwn^T'.I '^  a^;T'"-^' 
has  become  verv  in»-k  r«^.      j  •  *'  *™  *-   ►■«•<«■ 

with  the  i.f,  V  Jat^l^wr     r  ""■  •"'■  ''"'■"«  ''^  '-"-"o" 
w«  tlie  blood  now  flows  from  above  downward.     The 


i      I 


36a 


DEvzix>nfBU)T  (a  ikb  vExotw  synxM 


left  umbilical  now  forms  the  otAy  rovie  for  the  return  of  Wood  from 
the  placenta,  and  appears  to  be  the  direct  continuation  of  the  ductus 
venosus  (Fig.  i6i,  C),  into  which  open  the  hepatic  veins,  returning 
the  blood  distributed  by  the  portal  vein  to  the  substance  of  the  liver. 
Returning  now  to  the  posterior  cardinal  veins,  it  has  been  found 
that  in  the  rabbit  the  branches  which  come  to  them  from  the  mesen- 
tery anastomose  longitudinally  to  form  a  vessel  lying  parallel  and 
slightly  ventral  to  each  cardinal.    These  n.:iy  be  termed  the  sub- 


A  B 

TVS.  162.— DUOMMS  iLtDSntATINO  XHK  DlVItOPMKNT  01  imt  iHntUOI  VjKA  CAVA. 

The  cardinal  veins  and  ductm  venosa  are  black,  the  subcardinal  system  blue, 
and  the  supracardinal  yellow.  «,  ccvonary  sinus;  A/,  ductus  venosus;  if,  iliac  vein; 
f,  renal;  s,  internal  si)crmatic;  scl,  subclavian;  5r,  suprarenal;  va,  azygos;  vha,  hetni- 
azygos;  vi,  inoominate;  v;,  internal  jugular. 

cardinal  veins  (Lewis),  and  in  their  earliest  condition  they  open  at 
either  end  into  the  corresponding  cardinal,  with  which  they  are  also 
united  by  numerous  cross-branches.  Later,  in  rabbits  of  8.8  mm., 
these  cross-branches  begin  to  disappear  and  give  place  to  a  large 
cross-branch  situated  immediately  below  the  origin  of  the  superior 


al  which  is  anterioTtole^  ^^'^°''°^*'^  "■«'*' *"»«»«li»- 
and  unites  withTdtci  v^^rXr^'  T'"'^'^  ^°'''«- 
open  into  that  vessel  ^^16!^^^  ""^  ""^  ^'^'^'^  ^«'»» 
=«*nal  immediately  aWeL:'  !-  '^""'°  °'  ^"^^  P<«'«rior 

«tes,  so  that  aU  lie  bl^'^eVSTtl  "^  "•"'  ^'=''"  ''^«- 
cwdinals  is  letumed  toX  wt   ^       P°'""°''  P°"'°°^  «'  'J^ 

When  this  is  accompMshS  TZ  "^""^^  ^^^  venosus. 
<«sappear,  while  the^rto^a^vHreC'"""'''^'"'^"'--'' 
"St,  greatly  diminishedin  s^I       I         ^"^  '^''o^^onnection  per- 

walls  empty  intoTrpls^er  "^  f^,^''  Posterior  abdominal 
region  of  the  kidney  o?eachidelT'  ".""'''  ''''^^  ^°™'  "^  '^^^ 
opens  at  either  e  Jemity  nto  the   '°T""^  anastomosis  which 

thusbecomessnrr^S^bvavlt'^  'u'^"^''^'-  '^^  "«'«^ 
••s  Wed  by  the  newloiituH  "T  "^' '*•' '*°'^' "-"'^  °f -h'^h 

tanned  the  4„..i'Z'SXr„'d"^^^  '"^  ^-" 

ventral  hmb  is  formed  bv  a  nL^       7  I     ^nntington  ,  while  the 

r62,B).  St.-Ularttt»trE:  tfrr^'^*^'  (^'K- 
dorsal  supracardinal  UZl^T.^nV^T"^  '^  '^^ 
posterior  cardinal     An  .n...  '^         "*  '^^  "o"*  primitive 

right  and  left  c^'i:uf.ttrrT.'"'"  "'"^  '"'^n  'he 
them  (Fig  16.  ^  TnH  ^I       ^      "'""'"  '^'  '""^  ^""^  open  into 

spermatic  vein  dtprrr^'  .''^  '"*'"«  "'  'h'^  -'«--' 
renal  vein,  persittag'^sThe  „„-""'''''''  °'  "'  ^  '"  ^°™-1  ^^  'he 
vein,  which'Thus  Tome  i^T.^,:^  ""'  f  -"=-'  ^P--tic 
•he  vena  cava  as  does  the  co^'nH  **  ''"'"  "'^'"  '"^'^'^'i  "^  '-'o 
body  (Fig.  16.,  c.rrjP^f  »«**"' of  the  right  side  of  the 
cardinals  at  th  p^int  wh^  tW  '*"*  '^'^'■"*"^  "P**  '"'^  '^e 
connection,  and  wC  the, owe  t^t'rr:'  '^  *^  "^^  "-- 
this  cr«ss-com>ection  fomj  tlpCl    '  '^^  r[""*'  '''""PP'^^^' 

^hich  consequently  rece^fh.'^T        '^     °'  ""=  '"^f'  '*'«''  ^«'"' 
4  cntiy  receives  the  left  suprarenal  (Fia   .ft->  r-. 


(        i 


ii  11 


364 


DEVXU>niEMT  or  IBS   VXNOUS  SYSTXK 


The  observations  upon  which  the  above  description  is  based 
have  been  made  upon  the  rabbit,  but  it  seems  probable  from  the 
partial  observations  that  have  been  made  that  simihir  changes 
occur  also  in  the  human  embryo.  It  will  be  noted  from  what  has 
been  said  that  the  inferior  vena  cava  is  a  composite  vessel,  consisting 
of  at  least  four  elements:  (i)  the  proximal  part  of  the  ductus  venosus; 
(2)  the  anterior  part  of  the  right  subcj'-dinal;  (3)  the  right  supra- 
cardinal;  and  (4)  the  posterior  part  of  tho  nwht  cardinal. 

The  complicated  development  of  the  ■  rv  .  vena  cava  naturally 
gives  rise  to  numerous  anomalies  of  the  \  j'  due  to  inhibitions  of  ite 
development.  These  anomalies  affect  espt  ally  the  post-renal  portion,  a 
persistence  of  both  cardinals  (interpreting  the  conditions  in  the  terms  of 
what  occurs  in  the  rabbit)  giving  rise  to  a  double  post-renal  cava,  or  a 
persistence  of  the  left  cardinal  and  the  disappearance  of  the  right  to  a 
vena  cava  situated  on  the  left  side  of  the  vertebral  column  and  crossing 
to  the  right  by  way  of  the  left  renal  vein.  So,  too,  the  occurrence  of 
accessory  renal  veins  passing  dorsal  to  the  ureter  is  explicable  on  the 
supposition  that  they  represent  portions  of  the  supracardinal  system  of 
veins. 

It  has  already  been  noted  that  the  portions  of  the  posterior 
cardinals  immediately  anterior  to  the  entrance  of  the  renal  veins 
disappear.  The  upper  part  of  the  right  vein  persists,  however,  and 
becomes  the  vena  azygos  of  the  adult,  while  the  upper  portion  of  the 
left  vein  sends  a  cross-bianch  over  to  unite  with  the  azygos  and  then 
separates  front  the  coronary  sinus  to  form  the  vena  hemiazygos.  At 
least  this  is  what  is  described  as  occurring  in  the  rabbit.  In  the  cat, 
however,  only  the  very  uppermost  portion  of  the  rifjht  posterior 
cardinal  persists  and  the  greater  portion  of  the  azygos  and  perhaps 
the  entire  hemiazygos  vein  is  formed  from  the  prerenal  portions  ol 
the  Ripracardinal  veins,  the  right  one  joining  on  to  the  small  per- 
sisting uppw  portion  of  the  vight  posterior  cardinal,  while  the  cross- 
connection  between  the  heniiazygot  and  azygos  represents  one  of  the 
originally  numerous  cro^-'-onnections  between  the  supracardinals. 

The  ascending  lumbar  veins,  frequently  described  as  the  commence- 
ments of  die  azygos  veirs,  are  in  realiii  acsndary  formations  developed 
by  die  aii»to«o8es  o<  ^teriorly  and  po»t«riorly  directed  branches  of  the 
lun^bari 


MV.10PMENT  OF  IHE   V.NOCS  SVmK  ,65 

,^    '^^I^J^^'foP'^  Of  II":  veins  of  the  Limbs.-The  development  of 
the  bmb  ve,„s  of  .he  human  emb.70  requires  fur.herT,^SSon 

,n  1S.>    T^""""  °'  "'^'  '^  ^°^  *'■"»  -ha.  has  been  oZ^^ 
m  rabb,.  embryos  it  may  be  presumed  .hat  .he  changTlhich  ta^ 

tHe  cephaltc  vetn  of  .he  aHuIt  on,i     .  :.  ,     "^'"^""ng 

r  nui  uie  aauit,  and  .  ;  its  appearance  the  diffibil  win« 

StTn"  ?hT/r  '*"=  •'"""'^  "'^^-"-  ^o- -ni^^ 
iTs  pii,  L         ^'"°"  "'  '•>•=  P""""^  ^^^  --»  disappears. 

develops  wWi    ,    "'."?'  "^  con.inua.ion,  ,hc  ulnar  vein,  are 
cephairthlh     "'       f  """'  '''^^'^'"P^  "^  "^  °»'«^'"^'h  from  .he 

SuXSh- Te™''°"  ^'"^  ""^  ™^'  ^"«"-  ^o-'"^ 

In  .he  lower  limb  a  primary  fibular  vein,  exactly  comparable  to 

the  pnmaty  ulnar  of  the  arm,  surrounds  .he  dis.al  bo'rderXti! 

cardi"? v!^^?  f  T""'  '""•^"  '"  °f*"  -'h  'he   pos.erior 
cTsde  alh  r        development  i.  the  lower  limVdiffers 

CO.  ..de-ably,  however,  from  that  of  the  upper  limb.    From  the  pri- 
mary fibular  vein  an  anterior  tibial  ..in  grows  ou.,  whicr  r«:eives 

meS:  hT^'n-n'"  ''r^^'  ^'  ^™-  '^^^  Po^tenofcrdS 
Sw"  down  t^  ,"';'"';'r  P""^  «'"'^'  °P^»=  '"">  ■■'-  -  vein 
E  1  ,h  f  ','"*'  ^^  "^^  '"«■  '°™'°8  'he  long  saphenous  vein, 
trom  thi.  .he  femoral  veiu  is  formed  and  from  it  the  posterior  tibial 
vem  .s  continued  down  the  leg.  An  anastomosis  is  foSdTetwe  n 
he  femoral  and  the  primary  fibular  veins  at  the  level  of  2  taee  a^ 
the  prox,mal^ortion  of  the  latter  vein  then  becomes  greatly  r^duc^ 

S:!;;:.'.:;;  '"^"'''"  '"^"■"^'  ''""''■'  '•^  '"^  ^-"  Vnous^s 

ThePuln^ry  Veins.~Th.  development  of  .he  pulmonary  veins 


366 


1HX  IKUL  CnCDlATIOM 


has  already  been  deticribed  in  connection  with  the  development  of 
the  heart  (see  p.  334). 

The  Fetal  CiraUatum.—Dm'mg  fetal  life  while  the  placenU  is 
the  sole  oigan  in  which  occur  the  changes  in  the  blood  on  which  the 


1 


The  Fetal  Cikcthjvtion. 
00,  Aorta;  a.pu.,  pulmonarv  artery;  au,  umbilical  arteiv;  da,  ductus  uteriosus; 
dv,  ductus  venosus;  itU,  intestine;  vci  and  vcs,  inferior  and  superior  vena  cava;  vh, 
hepatic  vein;  vp,  vena  ports;  v.pu,  pulmonary  vein;  tw,  umbilical  vrin. — (From 
KoUjHonn.) 

nutrition  of  the  embryo  depends,  the  course  of  the  blood  is  neces- 
sarily somewhat  different  from  what  obtains  in  the  child  after  birth. 
Taking  the  placenta  as  the  starting-point,  the  blood  passes  along  the 


until  h  caches  the  W  H^f  o^^ir^h  '^»'*'>(-P-3«) 
the  umbilical  and  vitelUne  vefnT.  Tf-  '^"^ '""'"""^e*  between 
substance  of  the  IW^Z^ZTTL^  ""'•'"^  '^"^''"*' '"« 
vena  cava,  while  the  «Z^^    ^  •*'"  ^"''"'  "«°  'he  inferior 

'o  the  cav;,  th  J^^!^  1"  P*"^  °» 'trough  the  ductus  venosu. 
blood.  wh,;e  pu";ls  oM^^r"!^^^^^  ™s 

blood  returning  f„,mrrntifv'y  "^"'"^  ''^  ""'«««  """  the 
■■n«  into  the  n^ht  venttcl  Whe  F  »T'  "  ^'''"""^  *«"»  P»«- 
to  the  foramen  ovale  a^H,h  ''!.^""»='"«°  ^"'ve,  which  directs  it 
thence  to  the  "ft  V  S  t^"L  '"'^K  t""  '■"'°  "«=  '"="  "«- 

The  blood  wh^^^t  •£  Ttt  rrT""  ''°"^- 

extremities  is  returned  bv  Tl„T-  ^""''  ""'''  """^  "PP« 

atrium,  but  this  d^^diL  ^  '^  '  """^  ''^''  *'^  '"'o  the  ri^t 
of  the  annul^;  ^euS  ZtiT'TT'  """"» '^  "«'  "'^^ 
right  ventncle  without  mi"!  ^"an;!"".  """^  "'"^"^  '°  ""^ 
returning  by  way  of  the^    *      ^  *^'"  *'"'™'  '''th  the  blood 

this  bloc^  pLrout  by  h'puta?"'-  ''"""  *•'  "«"'  ^^"'"•''« 
period  are  c^aps^  and  n  n^i^H  v'^  ''"''^= ''"'  ''"^  '""'P  »t  this 
ofblood,andsoWs^2moarf  °'°.'^'"'''"y«^'''«"°u«t 
the  systemic  aom  mXT-rT°'*''*'"^'^''«"="°^"^i°to 
point  where  thet  t3vil„  J  ^"""'*'  '"'^  J"''  •^'"'^  the 
onward  the  ao^  co2t.  .  "^  "  ^'^''^  '''^-  F""»  this  point 
to  the  walls  of?e  ^117^"""'  "'^'  """^  "^''^  '^  ''-tributed 
inal  visce^,  th™^  2' *?r  '"'  *° ''''  '""'^ '"«'  '''^°«- 
gastn-c  arte;-es  anTsl^ rgTL  t^t  p^er'"*  °^  '"  '"«  ''^■ 

is  bS^^  uV^n-'rSVarrforamr'  "!  '^-^  '^'"'  -""^t'-  »"<«  it 
between  the  inferior  vena  4va  and X  leirt'  •"  ^'^i^'?"^  *  """"^tion 
nght  ventricle  receives  onirthe  b^oo Jrl  ^'"'""- .  "  '""  ""e-^t  the 
cava  superior,  while  the  feft  recei^l,  'S^'?^?  '"  '^^  '*?«  ''^  the  vena 
ava  together  with  what  retims  bv  fhl  i  '""'  '"''  **  '"'^rior  vena 
therefore,  expect  that  the  caZi^  .  !  Pulmonary  veins.  One  would 
would  in  thrfetusbe  I«.  fk     ^u  ""**  Pressure  of  the  right  ventride 

.^ntlyfave:Ug«edl'?ues^onfnTn,h'  '"">"-    ^°'"'^"    ^^o^ 
^^  the  capacit^-es  and  Tr^^r^ir^Z'^:^:^-:^^-^^^^ 


^ 


1 

i 

' 

■ 

' 

1 

i  ^ 

MKioconr  ifsouiriON  test  chait 

(ANSI  and  ISO  TEST  CHART  No.  21 


1.0 


I.I 


IM 


lit 


1.8 


^1^1^ 


^  /APPLIED  IM^GE    Inc 

^F  1653  East  Uain  StrMl 

RjS  RochxUr.  Hum  rork        14609       USA 

^B  (716)  482 -0300- Phorw 

jg  (716)  2Be  -  59B9  -  Fax 


368 


DEVELOPMENT  OF   THE    LYUFHATIC   SYSTEM 


maintains  that  the  foramen  ovale  is  actually  a  connection  between  the 
two  atria.  That  is  to  say,  he  holds  that  there  is  an  actual  mingling  of  the 
blood  from  the  two  veme  cava;  in  the  right  atrium,  whence  the  mixed 
blood  passes  to  the  right  ventricle,  a  certain  amount  of  it,  however, 
passing  through  the  foramen  ovale  and  so  to  the  left  ventricle  to  equalize 
the  deficiency  that  would  otherwise  exist  in  that  chamber  owing  to  the 
small  amount  of  blood  returning  by  the  pulmonary  veins.  According 
to  this  view  there  would  be  no  difference  in  the  quality  of  the  blood  distri- 
buted to  different  portions  of  the  body,  such  as  is  provided  for  by  the 
current  theory;  all  the  blood  leaving  the  heart  would  be  mixed  blood  and 
m  favor  of  this  view  is  the  fact  that  starch  granules  injected  into  either 
the  superior  or  the  inferior  vena  cava  in  living  pig  embryos  were  in  all 
cases  recovered  from  both  sides  of  the  heart. 

At  birth  the  lungs  at  once  assume  their  functions,  and  on  the 
cutting  of  the  umbilical  cord  all  communication  with  the  placenta 
ceases.  Shortly  after  birth  the  foramen  ovale  closes  more  or  less 
perfectly,  and  the  ductus  arteriosus  diminishes  in  size  as  the  pul- 
monary arteries  increase  and  becomes  eventually  converted  into  a 
fibrous  cord.  The  hypogastric  arteries  diminish  greatly,  and  after 
they  have  passed  the  bladder  are  also  reduced  to  fibrous  cords,  a  fate 
likewise  shared  by  the  umbilical  vein,  which  becomes  converted 
into  the  round  ligament  of  the  liver. 

The  Development  of  the  Lymphatic  System.— Concerning 
the  development  of  the  lymphatic  system  two  discordant  views 
exist,  one  (Sabin,  Lewis)  regarding  it  in  its  entirety  as  a  direct 
development  from  the  venous  system,  while  the  other  (Huntington, 
McClure)  recognizes  for  it  a  dual  origin,  a  portion  being  derived 
directly  from  the  venous  system  and  the  rest  from  a  series  of  mesen- 
chymal spaces  developing  in  relation  to  veins  but  quite  unconnected 
with  them. 

The  portion  of  the  system  concerning  which  harmony  prevaib 
is  that  which  forms  the  connection  with  the  venous  system  in  the 
adult  and  constitutes  what  in  the  embryo  is  termed  the  jugular 
lymph  sac.  In  the  early  stages  of  development  a  capillary  network 
extends  along  the  line  of  the  jugular  veins,  communicating  with 
them  at  various  points.  In  embryos  of  lo  mm.  a  portion  of  this 
network,  on  either  side  of  the  body,  becomes  completely  separated 


DEVEtOPMKNT  OF  IHE  LYMPHATIC  SVSIEM  .fig 


FlO.    164.— DiAOIAHS  SHOWINO  THB  AxBANOEln-KT  np  ,™.   r 

ACV.      ,     ^°^""<»°M^n°^:Cm°'(Br,„^^f'"™  VESSEL" 
fCVi  cardinal  ve m;  Pijy.  nnsterior  IvmrTl,  „     PSJ^'.'"'  branch  to  ocsophainS- 


(Fig.  164,  ^iH),  and  this,  gtiU  later, 


makes 


ayo 


DEVELOPMENT   OF  THE    LYIDHATIC   SYSTEM 


new  connection  with  the  jugular,  the  opening  being  guarded  by  a 
valve.  This  communication  becomes  the  adult  communication  of 
the  thoracic  duct  or  right  lymphatic  duct  with  the  venous  system, 
but  the  sac  itself,  as  development  proceeds,  becomes  divided  into 
smaller  portions  and  gives  rise  to  a  number  of  lymph  nodes. 

A  similar  pair  of  lymph  sacs  also  develop  in  relation  to  the 
sciatic  vein,  but  their  exact  mode  of  origin  is  uncertain.  In  embryos 
of  20  mm.  venous  plexuses,  similar  to  the  jugular  plexuses  of 
younger  stages,  are  found  accompanying  the  sciatic  veins,  and  a 
little  later  there  are  found  in  the  same  region  a  pair  of  posterior  or 
sciatic  lymph  sacs  (Fig.  164,  PLH),  which,  like  the  jugular  sacs, 
later  give  rise  to  a  series  of  lymph  nodes.  At  about  the  same  stage 
of  development  a  re/f0/>«rt/(m«a/  sac  (Fig.  165,  Lsr)  is  also  formed  in 
the  root  of  the  mesentery  cranial  to  the  origin  of  the  superior  mesen- 
teric artery,  and  this,  too,  later  gives  rise  to  a  plexus  of  lymphatic 
vessels  in  connection  with  which  the  mesenteric  lymphatic  nodes 
develop.  This  last  sac  is  much  more  pronounced  in  the  pig  embryo 
than  in  man,  and  in  that  form  it  has  been  found  to  have  its  origin 
from  a  capillary  network  that  separates  from  the  renal  veins 
(Baetjer). 

There  are  thus  formed  five  sacs,  all  of  which  are  associated  with 
the  formation  of  groups  of  lymphatic  nodes,  and  in  the  case  of  one 
pair  at  least  it  is  agreed  that  they  are  directly  developed  from  venous 
capillaries.  It  is  in  connection  with  the  remaining  sac  and  espe- 
cially with  the  formation  of  the  thoracic  duct  and  the  peripheral 
lymphatics  that  the  want  of  harmony  referred  to  above  occurs. 
The  first  portion  of  the  thoracic  duct  to  appear  is  the  cistema  chyli, 
which  is  found  in  embryos  of  23  mm.  in  the  region  of  the  third  and 
fourth  lumbar  segments,  in  close  proximity  to  the  vena  cava  (Fig. 
165,  Cc).  After  its  appearance  the  rest  of  the  thoracic  duct  develops 
quickly,  it  being  completely  formed  in  embryos  of  30  mm.,  and  it  is 
interesting  to  note  that  at  this  stage  the  duct  is  paired  in  its  caudal 
portion,  two  tnmks  passing  forward  from  the  cistema  chyli,  the 
right  one  passing  behind  the  aorta  and  uniting  with  the  left  after  it 
has  entered  the  thorax. 


DEVELOPMENT  OF  IHE   lYMPHATlC  SYSTEM  371 

The  mode  of  origin  of  the  duct  has  not  yet  been  made  out  in 
human  embryos.  In  the  pig  and  rabbit  isolated  spaces  lined  with 
endothelium  occur  along  the  course  of  the  duct,  but  without  com- 
m,micat.ng  with  it,  and  the  fact  that  some  of  these  showed  com»ec- 
tjon  with  the  neighboring  azygos  veins  gave  basis  for  the  view  that 
they  were  the  remains  of  a  venous  capillary  plexus  from  which  the 
duct  had  developed.    It  is  possible,  however,  that  the  duct  is  formed 


Sac  *™  IHE  'c^'«»Tc™™iS'0™f  °'  "^  R^ope«toneal  Ly»h 
lumtar  vertebra;  w„  fir,,  „crii  y^A'^X-TJ^^SMl.)         "™  '"'"""•  "'■■  ^' 

by  the  union  of  outgrowths  from  the  cisterna  chyli  and  jugular  sac, 

lid  th'TA'  """  "'".'^  '  "^'"^^'-^^  °^«'<^  venouf  system 
provided  that  the  cistema  chyli  is  formed  in  the  same  way  as  the 

jugular  sac.    Huntington  and  McClure,  however,  maintain  that  it 


J7a 


DEVElOPlffiNT  OF  THE   LWFHATTC  sySTElt 


is  formud  by  the  fusion  of  spaces  appearing  in  the  mesenchyme 
immediately  exicmal  to  the  intima  of  degenerating  veins;  hence  the 
spaces  are  termed  extraintimal  spaces.  These  at  first  have  no 
endothelial  lining  and  they  are  never  in  connection  with  the  lumina 
of  the  veins.  They  are  perfectly  independent  structures  and  any 
connections  they  may  make  with  the  venous  system  are  entirely 
secondary.  This  mode  of  origin  from  extraintimal  spaces  is  not 
confined  to  the  thoracic  duct,  according 
to  the  authors  mentioned,  but  is  the 
method  of  development  of  all  parts  of 
the  lymphatic  system,  with  the  exception 
of  the  jugular  sacs.  According  to  the 
supporters  of  the  direct  venous  origin 
the  peripheral  lymphatic  stems  develop, 
like  blood-vessels,  as  outgrowths  from 
the  stems  already  present. 

Lymph  nodes  nave  not  been  observed 
in  human  embryos  imtil  toward  the  end 
of  the  third  month  of  development,  but 
they  appear  i;i  pig  embryos  of  3  cm. 
Their  unit  of  structure  is  a  blood-vessel, 
breaking  up  at  its  termination  into  a 
leash  of  capillaries,  aroimd  which  a  con- 
densation of  lymphocytes  occurs  in  the 
mesenchyme.  A  structure  of  this  kind 
forms  what  is  termed  a  lymphoid  follicle 
and  may  exist,  even  in  this  simple  condition,  in  the  adult.  More 
frequently,  however,  there  are  associated  with  the  follicle  lymphatic 
vessels,  or  rather  the  follicle  develops  in  a  network  of  lymphatic 
vessels,  which  become  an  investment  of  the  follicle  and  form  with  it  a 
simple  lymph  node  (Fig.  166).  This  condition  is,  however,  in  many 
cases  but  transitory,  the  artery  branching  and  collections  of  lym- 
phoid tissue  forming  around  each  of  the  branches,  so  that  a  series 
of  follicles  are  formed,  which,  together  with  the  surrounding  lym- 
phatic vessels,  become  enclosed  by  a  connective-tissue  capsule  to 


FlO.     166. — DlAGKAU    OF    A 

PiuiuKY  Lyuph  Node  of  an 

EUBRYO   FXO    OF   8   CM. 

a,  Arteiy;  aid,  afferent  lymph 
duct;  da,  efferent  lymph  duct; 
/  folUde.— (5o6>ii.) 


DEVEIOPMENT  OF  IHE   lyMPHATlC  SYSTEM  ,73 


»74 


DEVEIOPIBNT   Or  THE    SPLEEK 


one  consisting  of  a  follicle,  composed  of  adenoid  tissue  with  a  central 
blood-vessel,  and  a  peripheral  blo<- '.  sinus  (Fig.  167). 

The  DeTelopment  of  the  Spleen.— Recent  studies  (Mall)  have 
shown  that  the  spleen  may  well  be  regarded  as  possessing  a  structure 
comparable  to  that  of  the  lymph  nodes,  the  pulp  being  more  or  less 
distinctly  divided  by  trabecule  into  areas  termed  pulp  cords,  the  axis 
of  each  of  which  is  occupied  by  a  twig  of  the  splenic  artery.  The 
spleen,  therefore,  seems  to  fall  into  the  same  category  of  organs  as 
the  lymph  and  iMemolymph  nodes,  differing  from  these  chiefly  in 
the  absence  of  sinuses.  It  has  generally  been  regarded  as  a  develop- 
ment of  the  mesenchyme  situated  between  the  two  layers  of  the 
mesogastrium.  To  this  view,  however,  recent  observers  have 
taken  exception,  holding  that  the  ultimate  origin  of  the  organ  is  in 
part  or  entirely  from  the  coelomic  epithelium  of  the  left  layer  of  the 
mesogastrium.  The  first  indication  of  the  spleen  has  been  observed 
in  embryos  of  the  fifth  week  as  a  slight  elevation  on  the  left  (dorsal) 
surface  of  the  mesogastrium,  due  to  a  local  thickening  and  vasculariza- 
tion of  the  mesenchyme,  accompanied  by  a  tUckening  of  the 
coelomic  epithelium  which  covers  the  elevation.  The  mesenchyme 
thickening  presents  no  differences  from  the  neighboring  mesenchyme, 
but  the  epithelium  is  not  distinctly  separated  from  it  over  its  entire 
surface,  as  it  is  elsewhere  in  the  mesentery.  In  later  stages,  which 
have  been  observed  in  detail  in  pig  and  other  amniote  embryos, 
cells  separate  from  the  deeper  layers  of  the  epithelium  (Fig.  168)  and 
pass  into  the  mesenchyme  thickening  whose  tissue  soon  assumes  a 
different  appearance  from  the  surrounding  mesenchyme  by  its  cells 
being  much  crowded.  This  migration  soon  ceases,  however,  and 
in  embryos  of  forty-two  days  the  coelomic  epithelium  covering  the 
thickening  is  reduced  to  a  simple  layer  of  cells. 

The  later  stages  of  development  consist,  of  an  enlargement  of 
the  thickening  and  its  gradual  constriction  from  the  surface  of  the 
mesogastrium,  until  it  is  finally  united  to  it  only  by  a  narrow  band 
through  which  the  large  splenic  vessels  gain  access  to  the  organ 
The  cells  differentiate  themselves  into  trabecuUe  and  pulp  cords 


MVELOPHENT   OF  THE   SPLEEN 


cm.  there  is  to  be  found  on  the  ven^sTS.  ^^  "'  ^''°"'  'S 
coccyx  a  small  oval  group  of  Dollnn!,  m  ?  °^  ""^  ^P^^  "^  'he 
the  surrounding  tiss^  by  a  me ifrh  'f  '  "'""^  '^P^^^'^^  ^o™ 
^ve-tissue  trabecule  mate'  ^rlXtTT'  '^'''''  ^°""- 
becomes  divided  into  lobules  and  7^hT  '°^''  ''^'^''  'hus 

supply,  derived  principallv  Trol^'      u      ^""^  '""^'  «  "ch  vascular 
penetrates  the  M^MAur  ''':"''''''' ''"'^'^''^^^y' 

which  itpresentsag'neTalresmblaT"  "'  '''""  '°"'^"'°"  '" 
It  has  generally  beerlpold  K  "  "  '  ^""^  "'  '^'"P'>  f""'"'^'-- 
part  derived  from  the  syXhel  t^'^ '=°"y8''«' «-«'ion  was  in 

-samegroupoforganL^t;ra;S.- -?-£:: 


!  I 


»76 


LITEKATUU 


work  on  its  development  (Stoerk)  tends,  however,  to  disprove  this 
view,  and  the  ganglion  seems  accordingly  to  find  its  place  among 
the  lymphoid  organs. 

LITERATURE. 

^'  eJ^'I^  "  ?"  *%°'^"  °'  ""  *'~"'"'^  ^  •""  *'  Tho""^''  Duct  In  Ih, 
Embiyo  Pig,"  Amer.  Jaum.  Anal.,  vm,  J908. 

E.  v*«  Beniden  ,„d  C.  J„i«:  "RKhwch^  .ur  I.  tomuiUon  d«  ™,«„  teul« 
cho  les  numuniftre.,"  Archiva  dt  Biolo,.,  v  18S4 

'■  ^m,*!^'  " °"  '^'  "'^ °' '^'' ^^"""y *""i" i" M.mm.1.,"  ^.o/. Ji«,i. 

'■  "™Z  hZ^^J'L,^"'"''"'"''  W"<imu,g  u„d  VarkUon  der  B.uch«rt„. 
»w.^  bd  da.  WirbeW.™,"  Ergeb.  Anal.und  Entwick.,  xvi,  ,go6 

inaus  btim  MaiKhen,"  Anat.  Heflt,  xxxvi,  1908 

BMung  in  Mil^,  Leber  und  Lyn.pluirU.en,"  Dcul^ch.  Arch.  f.  Wn.  M«l.,  xcn, 

^'  ^f:!;!l."'J'°,'T""i""  °°  '"''^  '^"'"'^  Lymplwtic.  in  tl«  T.ii  of  the  Frog 
1-arva,    Anal.  Record,  ni,  1909.  ^ 

^"  "^^'S!!;  "■^'.  ^'5'  r"™!"?"-"'  of  U.e  Aortic  Arche.  of  the  Cat,  wiU. 

Ve^T"     f,""""!  ""^''Veinsin  EariyPig  Embryo, and ie i^ of  U« 
Vena  Cava  Inferior,"  Amer.  Joun.  Anal.,  x,  1910 

*"n!r^r^^:.':^X.'''^'^"'"'^^''^'''«'-"''™^^^ 

"■  '^/yr\i,'°V,^°'"'°'""""  "*  *'  *<■"*■  C"^""  "<i  UmbiUcal  Vein., 

W  FEUxT- J,'^*"  t  ^■""^'"-g  "«  Lungenvene,"  Anal.  Hefle.  xt,  .9.0. 

Z*4  f "En'w,<ilungsge«:hichted.rRumpfarterien de. men«:lUich« Embryo." 
Jaorphol.  Jahrb.,  xti,  1910.  -^  ' 

*^'  ^I^^'T-I"?"  '^'/'"P"""'  °'  *«  1-ymplu.tic.  in  tl,e  SmaU  Intes-ine  of  the 
rig,    Amer.  Jaim.  Anal.,  ix,  1909. 

W.  His:  "Anatomie  menschUcher  Embryonen,"  Leipzig,  1880-18S, 

^«T^"''H''rt  ""'',™"'"'^°'^"P'"'-'^"  derhinterenGUedmasse 
F^fdf  r!?  A  ''f  Saugethiere,  nebst  Bemerkungen  fiber  die  Entwickiung  der 
Endiste  der  Aorta  ahdominalis,"  Morphol.  Jahrtmch,  xvi,  1890. 


^JntLATutx 


K  HoCHiTtrrui:  "p-i—  „    i.  '77 

O-  S.  HUWTINOTON  AND  C    F    U-    M^  "^ 

-  AS;^t£^?"- '  '"^--^  --  -  ^-;^ 
H-  ^^^^SSS- "^ '"  ""^'"-  ^^ — 

-•^^^:"J:     "^"•^•™"'-'-----. 

*-T.  L«wi3:  "Then™.!  ^""''"  iiinbiyos." 

F-  T.  Liwis:  "The  Fir..  V        L  "i""*!!  Embiyos," 

„    *^-rf.  m,  x^'  '^'"'  ^^-P^   ^^-"d.  U,  Rabbi,  and  K^  e„,        „ 

W.  A.  Locy:  "TheFiffi,      ^  e-  Embiyos,"  Amu. 

on  «..  CoS;rtr/«rC^""^™^^-'».<»  w,bc 
p      XXIX,  1906.  *""'  Vessels  in  other  Verleb«.„"'  "„,  Comment 


»78 


UTEBATDU 


C.  F.  W.  McCiDU:  "Tlw  DnrdopiMBt  al  Oh  Thandr  ud  Right  Lymplatk  Ducts 

in  the  Domcuic  Cat  (Fclb  Donuitk*),"  Anal.  Atmiftr,  xxni,  1908. 
C.  F.  W.  McCLintz:  "The  Eitn-lnllnul  Theory  ol  the  Development  of  the  McMnteik 

Lymphatics  In  the  Domnik  C»t,"  Vtrkandl.  Anal.  GneUsck.,  xxiv,  1910. 
C.  S.  Minot:  "On  a  Hitherto  Unrecognized  Form  of  Blood  CircuUtioo  without 
CepUUriei  In  the  Organs  o*  VcrtebraU,"  Proc.  BoiUmScc,  Nai.  IIUI.,  xxix,  1900. 
S.  MoLLln:  "Die  Blutbiidung  in  der  Embiyooalen  Leber  da  Menichen  und  der 

SiUgetlere,"  Ank.far  mUrcik.  Anal.,  ixxjv,  1909. 
A.  G.  Poblhan:  "The  Coune  at  the  Blood  thiiough  the  Fetal  Mammalian  Heut," 

Anal.  Kteord,  11,  1908. 
F.  Ruoan:  "The  Fifth  Aortic  Arch  of  Mammalian  Embryo*."  Amer.  Jam.  Ani., 

xii,  1913. 
E.  RlTTKBU:  "Sur  la  part  que  prend  I'cpitUllum  k  la  formation  de  la  boune  de 
Fabridus,  det  amygdalea  et  dei  plaquea  de  Peyer,"  Jcnrn.  da  fAnal.  il  da  la 
Pkytiot.,  XXIX,  1893. 
R.  Rmu:  "Some  Resulu  of  Recent  Inveatlgatlont  on  the  Mammalian  Heart," 

Anal.  Record,  11,  1908.  ^ 

C.  Rosi:  "Ilur  Entwlcklungigeichichte  del  Siugethlerherzeni,"  Itcrpkal.  Jakrhntk, 

XV,  i8«i. 
FioMNca  R.  Sabdi;  "On  the  Origin  of  the  Lymphadc  System  from  the  Veins  and 
the  Development  c(  the  Lymph  Hearts  and  Thoracic  Duct  in  the  Fig,"  Anm. 
J  own.  of  Anal.,  l,  1903, 
Flounci  R.  Sabin:  "The  Development  of  the  Lymphatic  Nodes  in  the  Pig  and 

their  Relation  to  the  Lymph  Hearts,"  Amir.  Jmtrn.  Anal.,  IV,  1905. 
FloiINCl  R.  Sabin:  "Further  Evidence  on  the  Origin  of  the  Lymphatic  Endothelium 

from  the  EndotheUum  of  the  Blood  Vascular  System,"  Anal.  Kocord,  11,  1908. 
FlolINCl   R.  Sabin:   On  the  Development  of  the  Lymphatic  System  in  Human 
Embryos  with  a  Consideration  of  the  Morphology  of  the  System  as  a  Whole  " 
Amer,  Jonrn.  Anal.,  ix,  1909. 
Flounce  R.  Sabin:  "A  Critical  Study  of  the  Evidence  Presented  in  Several  Recent 

Articles  on  the  Development  of  the  Lymphatic  System,"  Anal.  Stcord,  v,  1911. 
F.  Saxu:  "Ueber  die  Entwicldung  und  der  Bau  normaler  LymphdrUsen  und  die 

Entsehung  der  loten  und  wcisaen  BlutkSrperchen,"  Anal.  Hefle,  vi,  1896. 
H.  SCHIIDDI:  "Die  Entstehung  der  enten  embtyonalen  BluUellen  des  Menschen," 

Fotia  kamatot,  iv,  1907, 
P.  StShx:  "Ueber  die  Entwlcklung  der  Darmlj-mphknStchen  und  ttber  die  Rflck- 

bildung  von  Daimdrttsen,"  ArckivfSr  mikrosk.  Anal.,  Ll,  1898. 
O.  VAN  DEB  SiracHT:  "Nouvelles  recherches  sur  la  genise  des  gtobules  rouges  et  des 

globules  blincs  du  sang,"  Arckiva  da  Biolog.,  xa,  189J. 
O.  VAM  DER  Sibjcht:  "De  la  premiere  origine  du  sang  et  des  capillairea  sanguins  dans 
I'aiii  vasculau^  du  Lapin,"  CompUs  Rendui  do  la  Soc.  do  Biolog.  Paris,  Sit.  10, 
n,  1895. 
O.  Stoeee:  "Ueber  dw  Chiomreaktion  der  Glandula  coccygea  und  die  Besiehung, 
dieser  Drttse  ziim  ^   mvus  sympathicus  "  Arch,  far  mikroskof,  .4nat.,  I,xix,  1906. 
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idorfhol,  Jahrbuck,  xxx,  1903. 


UIXIATDU 

•;pr^; :- — -  ™.  «.o™™,  .,,.,^.  z 


CHAFIER  X 

THE  DEVELOPMENT  OF  THE  DIGESTIVE 
TRACT  Ain)  GLAITDS. 

The  greatest  portion  of  the  digestive  tract  is  formed  by  the  con- 
striction ofif  of  the  dorsal  portion  of  the  yolk-sac,  as  jhown  in  Fig.  52, 
the  result  being  the  formation  of  a  cylinder,  closed  at  either  end, 
and  composed  of  a  layer  of  splanchnic  mesoderm  lined  on  its  inner 
surface  by  endoderm.  This  cylind*  is  termed  archenterm  and  has 
connected  with  it  the  yolk-stalk  and  the  allantois,  the  latter  com- 
municating with  its  somewhat  dilated  terminal  portion,  which  also 
receives  the  ducts  of  the  primitive  kidneys  and  is  known  as  the 
cloaca  (Fig.  170). 

At  a  very  early  stage  of  development  the  anterior  end  of  the 
embryo  begins  to  project  slightly  in  front  of  the  yolk-sac,  so  that  a 
shallow  depression  is  formed  between  the  two  structures.  As  the 
constriction  of  the  embryo  from  the  sac  proceeds,  the  anterior  portion 
of  the  brain  becomes  bent  ventrally  and  the  heart  makes  its  appear- 
ance immediately  in  front  of  the  anterior  surface  of  the  yolk-sac, 
and  so  the  depression  mentioned  above  becomes  deepened  (Fig.  169) 
to  form  the  oral  sinus.  The  floor  of  thisj  lined  by  ectoderm,  is 
immediately  opposite  the  anterior  end  of  the  archenteron,  and,  since 
mesoderm  does  not  develop  in  this  region,  the  ectoderm  of  the  sinus 
and  the  endoderm  of  the  archenteron  are  directly  in  contact,  forming 
a  thin  pharyngeal  membrane  separating  the  two  cavities  (Fig.  169,  pm) 
In  embryos  of  2.15  mm.  this  membrane  is  still  existent,  but  soon  after 
it  becomes  perforated  and  finally  disappears,  so  that  the  archenteron 
and  oral  sinus  become  continuous. 

Toward  its  posterior  end  th  archenteron  comes  into  somewhat 
similar  relations  with  the  ectoderm,  though  a  marked  difference  is 
noticeable  in  that  the  area  over  which  the  cloacal  endoderm  is  in 

280 


BKVELOPMENT   OP  THE    DIGESTIVE   TBACT  281 

contact  with  the  ectoderm  to  form  the  cloacol  membrane  (Fig.  170,  cm) 
hes  a  little  m  front  of  the  actual  end  of  the  archenteric  cylinder,  the 
portion  of  the  latter  which  lies  posterior  to  the  membrane  forming 
what  has  been  termed  the  postanal  gut  (p.an).    This  diminishes  in 
sue  duxmg  development  and  early  disappears  altogether,  and  the 
pouch-hke  fold  seen  in  Fig.  170  between  the  intestinal  portion  of  the 
archenteron  and  the  allantoic  stalk  (a/)  deepening  until  its  floor 
comes  into  contact  with  the  cloa- 
cal   membrane,   the  cloaca  be- 
comes divided  into  a  ventral  por- 
tion,  with  which  the  allantois 
and  the  primitive  excretory  ducts 
(w)  are  connected,  and  a  dorsal 
portion  which  becomes  the  lower 
end  of  the  rectum.    This  latter 
abuts  upon  the  dorsal   portion 
of  the  cloacal  membrane,   and 
this  eventually  ruptures,  so  that 
the  posterior  communication  of 
the  archenteron  with  the  exterior 
becomes  established.     This  rup- 
ture, however,  does  not  occur  un- 
til a  comparatively  late  period  of 
development,  until  after  the  em- 
bryo has  reached  the  fetal  stage; 

nor  does  the  position  of  the  membrane  correspond  with  the  adult 
anus,  since  later  there  is  a  considerable  development  of  mesoderm 
around  the  mouth  of  the  cloaca,  bulging  out,  as  it  were,  the  sur- 
rounding ectoderm,  more  especially  anteriorly  where  it  forms  the 
laige  genital  tubercle  (seeChapterXIII),  and  posteriorly  where  it  pr<v 
duc^  the  anal  tubercle.  This  appears  as  a  rounded  elevation  on 
each  side  of  the  median  line,  immediately  behind  the  cloacal  mem- 
braneand  separated  from  the  root  of  the  caudal  projection  by  a  de- 
pression the  precaudal  recess.  Later  the  two  elevations  unite  across 
the  median  Ime  to  form  a  transverse  ridge,  the  ends  of  which  curve 


FlO.     l6g.— RrcoNSTRUCTlOK     OF    THE 

Antewor  Postioh  or  an  Eubcto  of  j  i  c 
uu.  ^ 

oi.  Aortic  bulb;  »,  heart;  o,  auditory  cap- 
sule;o#,  optic  evi«ination;«i»,pharyii|Kal 


383 


DIOESTIVZ  TKACT  AMD  OLAMDS 


forward  and  eventually  meet  in  front  of  the  original  anal  orifice. 
From  the  mesoderm  of  the  circular  elevation  thus  produced  the  ex- 
ternal sphincter  ani  muscle  is  formed,  and  it  would  seem  that  so 
much  of  the  lower  end  of  the  rectum  as  corresponds  to  this  muscle 
b  formed  by  the  inner  surface  of  the  elevation  and  is  therefore 
ectodermal.  The  definitive  anus  being  at  the  end  of  this  terminal 
portion  of  the  gut  is  therefore  some  distance  away  from  the  posi- 
tion of  the  original  cloacal  membrane. 


Fio.  17a — Reconstkuctioh  of  the  Hind  End  of  an  Ehbsyo  6.5  mi.  Loho 

«/,  AUantois;  b^  belly-stalk;  cl,  cloaca;  cm,  cloacal  membrane;  i,  intestine;  ft,  spinal 
cord;  ne,  notochord;  p-am^  postanal  gut;  w,  outgrowth  to  form  ureter  and  metenepnros; 
w,  Wolffian  duct.— (£«M.) 


It  will  be  noticed  that  the  digestive  tract  thus  formed  consists  of 
three  distinct  portions,  an  anterior,  short,  ectodermal  portion,  an 
endodermal  portion  representing  the  original  archenteron,  and  a 
posterior  short  portion  which  is  also  ectodermal.  The  differentia- 
tion of  the  tract  into  its  various  regions  and  the  formation  of  the 
various  organs  found  in  relation  with  these  may  now  be  considered. 


DEVEtOPmiNT  or  THE   MOUIH  MGIONS 

of  tJ?!J?'^'*T'?*  f  *^'  ^"""^  Reglon.-The  deepening 
of  the  oral  sinus  by  the  development  of  the  first  brancUal  arch  and 

oTthTXeV""  T  Tl'""^  "'"'  """^'  "^y  '••^  development 
o  It  •.  T  '^^^  ^"  '^'^^'^  (P-  99).  but.  for  the  sake 
of  contauuy  m  descnpUon,  the  latter  process  may  be  briefly  recalled. 
At  first  the  nasal  pits  communicate  with  the  oral  sinus  by  grooves 
bring  one  on  each  side  gf  the  f^ntfi-n^l  process,  but  by  [hfunion 
of  the  latter,  through  its  processus  globularis,  with  the  maxillary 
processes  these  communications  are  interrupted  and  the  floors  of 
rt>e  nasal  pits  are  sep^ated  from  the  oral  cavity  by  thin  bucco-nasal 
«.«W,  formed  of  the  nasal  epithelium  in  contact  with  that 

Wk  r  "T"^:  i"  '""'"^'''  "^  ''"'"'  '5  mm.  these  membranes 
break  through  and  disappear,  and  the  nasal  and  oral  cavities  are 
agam  m  communication,  but  the  communications  are  now  behind 
ZZ^^^  ^T'''^  """^  '°"'''""*  "^^  *^=  t«™«d  the  primiiive 
w^^^'     If         ""'"^  *'  *''  ''"«'=  '•"^  °°'-  ^°'"^^'  correspond 
with  the  adult  mouth  cavity,  since  there  is  as  yet  no  palate,  the  roof 
of  the  oral  cavity  bemg  the  base  of  the  skull.    From  the  maxillo- 
L tr*  r?^"'  °^l^'  upper  jaw.  shelf-like  ridges  begin  to  grow, 
be  ng  at  fim  directed  downward  so  that  their  surfaces  are  parallel 
with  .he  sides  of  the  tongue,  which  projects  up  between  them. 
Later,  however,  they  become  bent  upward  to  a  horizontal  position 
(Fig.  171)  and  eventually  meet  in  the  median  line  to  form  the  palate 
separaung  the  nasal  cavities  from  the  mouth  cavity.    All  that  por- 
Uon  of  the  original  oral  cavity  which  lies  behind  the  posterior  edge 
of  the  palatal  shelf  is  now  known  as  the  pharynx,  the  boundary 
between  this  and  the  mouth  cavity  being  emphasized  by  the  pro- 
longation backward  and  downward  of  the  posterior  angles  of  the 
palatal  she     as  ridges,  which  form  the  pharyngo-palatine  arche, 
{posterior  pilars  of  the  fauces).    The  nasal  cavities  now  communicate 
with  the  upper  part  of  the  pharynx  (naso-pharynx)  by  the  posterior 
cnoame.    The  palatal  processes  are  entirely  derived   from   the 
maxillary  processes,  the  premaxillary  portion  of  the  upper  jaw, 
Which  is  a  derivative  of  the  fronto-nasal  processes,  not  taking  part 
m  theu-  fomation.     Consequently  a  gap  exists  between  the  palatal 


384 


DEVELOFliXNT  OF  IHE   ICOUZH  KEOIONS 


shelves  and  the  premaxilte  for  a  time,  by  which  ;he  nasal  and 
mouth  cavities  communicate;  it  places  the  organ  of  Jacobson  (see 
p.  429)  in  communication  with  the  mouth  cavity  and  may  persist 
until  after  birth.  Later  it  becomes  closed  over  by  mucous  mem- 
brane, but  may  be  recognized  in  the  dried  skull  as  the  foramen 
incisivum  (anterior  palatine  canal). 

Occasionally  th»re  is  a  failure  of  the  union  of  the  palatal  plates,  the 
condition  known  as  cUJt  palaU  resulting.  The  inhibition  of  develofment 
which  brings  about  this  condition  may  take  place  at  different  stages,  but 
frequently  it  occurs  while  the  plates  still  have  an  almost  vertical  direction. 
Typically  cleft  palate  is  a  deficiency  in  the  median  line  of  the  roof  of  the 


Fio.  171.— Vaw  OP  THE  Roop  or  the  Oilil  Foss*  of  Embryo  showing  thi  Lip- 

OaOOVX  AMD  THE  FoUUTiaN  OP  THE  PALATE.— (ffjj.) 


mouth,  not  affecting  the  upper  jaw,  but  very  frequendy  it  is  combined 
with  the  defect  which  produces  hare-lip  (see  p.  100),  in  hich  case  the 
cleft  may  be  continued  through  the  upper  jaw  between  its  maxillary  and 
premaxillary  portions  on  eiUier  or  both  sides,  according  to  the  extent  of 
the  defect. 

At  about  the  fifth  week  of  development  a  downgrowth  of  epi- 
thelium into  the  substance  of  both  the  maxillary  and  fronto-nasal 
processes  above  and  the  mandibular  process  below  takes  place,  and 
the  surface  of  the  downgrowth  becomes  marked  by  a  deepening 
groove  (Fig.  171),  which  separates  an  anterior  fold,  the  lip,  from 
the  jaw  proper  (Fig.  172).    Mention  should  also  be  made  of  the 


MVElOPlffiNT  or  IHE  IKEIH  jgj 

'^^\^':Z'^^^^.t^''P-^''^  ^  --'^  '-^  ^-ed  in  the 
.^.n>e.b.a„eXtoutrh7.L"e5l^^^^^^ 

(seep.  399)  ^'"'  "  ""=  P"'"''«^  ^Y 

w..?,irrr;Hf  ,t^rei;t:eJvr  ^  r— 

develops  from  it  which  Lf^^t '« /"'•'"ed,  a  honzontal  outgrowth 

appears.    These  tooth-germs  reDreserrtll  •    •         ^^^^  ^«*'" 

oping  LneS  twf  ?  T   't !°  '"'^  ^''''  ""'^'  P*?'"*  ^^vel- 
These  r^nrlnt  .r  "  ^^'^''""^^  tooth-germs  are  formed. 

Ihese  represent  the  permanent  molars;  their  formation  is  much 


g8(i  DEVXLOnOCNT  Of  THE   TEKTK 

later  than  that  of  the  other  teeth,  the  germ  of  the  second  molar  not 
appearing  until  about  the  sixth  week  after  birth,  while  that  of  the 
third  is  delayed  until  about  the  fifth  year. 

As  the  tooth  germs  increase  in  size,  they  approach  nearer  and 
nearer  to  the  surface  of  the  jaw,  and  at  the  same  time  the  enamel 
organs  separate  from  the  dental  shelf  until  their  connection  with  it 
is  a  mere  neck  of  epithelial  cells.  In  the  meantime  the  dental  shelf 
itself  has  been  undergoing  degeneration  and  is  reduced  to  a  reticulum 


Fio      I73-T«ANSVnS£    SECTIONS    THIOUOH    TBI    LOWIR    JaW    OTOWDJO    IBT 
FO^TK)N  OF  ^Su.  SB.«  m  EhB«OS  OF  (A)    .7  MM.  AND  (B)  AP  »•.- 

which  eventually  completely  disapp':ars,  though  fragments  of  it  may 
occasionally  persist  and  give  rise  to  various  malformations.  With 
the  disappearance  of  the  last  remains  of  the  shelf,  the  various  tooth- 
germs  naturally  lose  all  connection  with  one  another. 

It  will  be  seen,  from  what  has  b  -n  said,  that  each  tooth-germ 
consists  of  two  portions,  one  of  which,  tiie  enamel  organ,  is  derived 
from  the  ectoderm,  while  the  other,  the  dental  papilla,  is  mesen- 


DEVZLOPMENT  OF  THE   TEETH 


>87 


chymatous  Each  of  these  gives  rise  to  a  definite  portion  of  the 
fully  formed  tooth,  the  enamel  organ,  as  its  name  indicates,  produc- 
ing the  enamel,  while  from  the  dental  papUIa  the  dentine  and  pulp 
are  formed. 

The  cells  of  the  enamel  organ  which  are  in  contact  with  the  sur- 
face of  the  papilla,  at  an  early  stage  assume  a  cylindrical  form  and 
become  arranged  in  a  definite  layer,  the  mamel  membrane  (Fig. 
173,  SEi)  while  the  remaining  cells  {SEa)  apparently  degenerate 
eventually,  though  they  persist  for  a  time  to  form  what  has  been 
termed  the  enamel  pulp.  The  formation  of  the  enamel  seems  to  be 
due  to  the  direct  transformation  of  the  enamel  cells,  the  process  begin- 
mng  at  the  basal  portion  of  each  cell,  and  as  a  result,  the  enamel 
consists  of  a  series  of  prisms,  each  of  which  represents  one  of  the 
cells  of  the  enamel  membrane.  The  transformation  proceeds 
until  the  cells  have  become  completely  converted  into  enamel 
pnsms,  except  at  their  very  tips,  which  form  a  thin  membrane,  the 
enamel  cuitcle,  which  is  shed  soon  after  the  eruption  of  the  teeth. 

The  dental  papillae  are  at  first  composed  of  a  closely  packed  mass 
of  mesenchyme  cells,  which  later  become  differentiated  into  connec- 
Uve  tissue  into  which  blood-vessels  and  nerves  penetrate.  The 
superficial  cells  form  a  more  or  less  definite  layer  (Fig.  173,  od) 
and  are  termed  odontoblasts,  having  the  function  of  manufacturing 
the  dentine.  This  they  accomplish  in  the  same  manner  as  that  in 
which  the  periosteal  osteoblasts  produce  bone,  depositing  the  den- 
toe  between  their  surfaces  and  the  adjacent  surface  of  the  enamel. 
The  outer  surface  of  each  odontoblast  is  drawn  out  into  a  number 
of  exceedingly  fine  processes  which  extend  into  the  dentine  to  occupy 
the  mmute  dentinal  tubules,  just  as  processes  of  the  osteoblasts 
occupy  the  canaliculi  of  bone. 

At  an  eariy  stage  the  enamel  membrane  forms  an  almost  com- 
plete investment  for  the  dental  papilla  (Fig.  173),  but  as  the  ossifi- 
cation of  the  tooth  proceeds,  it  recedes  from  the  lower  part,  until 
finally  it  is  confined  entirely  to  the  crown.  The  dentine  forming  the 
roots  of  the  tooth  then  becomes  enclosed  in  a  layer  of  cement,  which 
IS  true  bone  and  serves  to  unite  the  tooth  firmly  to  the  walls  of  its 


i  -M 


i66 


DKVSLOPIOENT  OF  IBB   TEETH 


socket.  As  the  tooth  increases  in  size,  its  extremity  is  brought 
nearer  to  the  surface  of  the  gum  and  eventually  breaks  through,  the 
eruption  of  the  first  teeth  usually  taking  place  during  the  last  half 
of  the  first  year  after  birth.    The  growth  of  the  permanent  teeth 


Fio.  1 73.— Section  throuoh  thk  First  Mouk»  Tooth  of  a  Rat,  Twiive  Davs  Old. 
Ap,  Periosteum;  E,  dentine;  Ep,  epidermis;  Od,  odontoblasts;  5,  enamel;  SEo 
and  SEi,  outer  and  inner  layers  of  the  enamel  organ;  SE,  portion  of  the  enamel  organ 
which  does  not  produce  enamel. — (vtm  Brunn.) 

proceeds  slowly  at  first,  but  later  it  becomes  more  rapid  and  pro- 
duces pressure  upon  the  roots  of  the  primary  teeth.  These  roots 
then  undergo  partial  absorption,  and  the  teeth  are  thus  loosened 


DEVllOWttNT  Of  IHE  TONOOT  ggg 

PUHUY  DiNTIIION. 

Median  inciwn «.i. .    „  i 

L.ter,lind«.n «th  to 8U. mon.h. 

FtatmoUm 8*  to  »  mo»th. 

Caninra....  Beginning  of  ad  y««r. 

Second  moUrs... '*!"?■■ 

3  to  3}  ytmn. 

The  teeth  of  the  lower  jaw  generaUy  precede  those  of  the  upper. 

Permanent  Dentition. 
First  molan . . 

Middle  inciMp,....;: r*'^- 

LateralindMn *'?''"'• 

Fint  premolar. S'?'""- 

Second  premolars..        'oth/ear. 

Canine.            1         "thyear. 

Secxmd  molar.  J    ijlh  to  14111  yean. 

Thiid  molan . 

'?">  to  4atl>  year^ 

theyLrrS.e*ri*oft Et^^y""^  '"''?  4  <">  •<> 
tional.    These  and  other  M«.l!»r?«~     7'        "  ""."""y  ?"'')''<•'><:• 

b.the«  teeth  ir.d.catetYt^efa?erdeSoi^A--S„r:^^^^^^^ 

t„n?i!  ."r'T*".*  °^  ^'  Tongue-Strictly  speaking,   the 
tongue  .s  largely  a  development  of  the  pharyngeal  Lon  of  Z 

ttftTtn  '"'  f'  ^"°""*""^  grows  fZard  into  T^L^i 
the  mouth.    In  embryos  of  about  3  mm.  there  may  be  seen  k  the 

t'hlt^t  a"n7' '''  ST  "'  I'^  '"""'•''  ^^''^-°  the'vStrends  o 
1m  t  J  K  '""'^  '''^"'•^^  ''"''^'  '^  *'»»"  funded  elevation 
wluch  has  been  termed  the  tuberculum  impar  (Fig.  174,  nXuZ 
at  one  n^e  believed  that  this  gave  rise  to  the  anferior  ^^on  of  T^ 
tongue,  but  recent  observations  seem  to  show  that  T reaches  Tb 
greatest  development  in  embryos  of  about  8  mm.,  after  wStS 
becomes  less  prominent  and  finally  unrecognizable.    Burbefore 


ago 


DBVILOPIBNT  OW  THE  TCMOCT 


this  occurs  a  swelling  appears  in  the  anterior  part  of  the  mouth  on 
each  side  of  the  median  line  (Fig.  174, 0.  and  these  gradually  increase 


FlO.    174.— FtOOK  0»  TH«  MODTH  AND  PhA«YNX  OF  AH  EllBIVO  OF  7.5  liH.,  ItOtl 
A   RlCONSTKUCnON. 

Cop.  Copula;  /,  (unula;  (,  swelling  lh«t  gives  rise  to  the  body  of  the  tongue;  Ti, 
tuberculum  Impar;  /-//f,  branchial  arches. 

in  size  and  eventually  unite  in  the  median  line  to  form  the  main 
mass  of  the  body  of  the  tongue.    They  are  separated  from  the 

neighboring  portions  of  the  first 
branchial  arch  by  a  deep  groove, 
the  alveolo-lingual  groove,  and  pos- 
teriorly are  separated  from  the 
second  arch  by  a  groove  which  la- 
ter becomes  distinctly  V-shaped 
(Fig.  17s),  a  deep  depression,  which 
gives  rise  to  the  thyreoid  body  lying 
at  the  apex  of  the  V.  Behind  the 
thyreoid  pouch  the  ventral  ends  of 
the  second  and  third  branchial  arches 
unite  to  form  an  elevation,  the 
copula  (Fig.  174,  cop),  and  from  this 
and  the  adjacent  portions  of  the 
second  and  third  arches  the  posterior  portion  of  the  tongue  develops. 
The  tongue  then  consists  of  two  distinct  portions,  which  even- 


FiG.    175. — The   Fioor   of   the 

PBAXYNX  of  an  EhBKYO    of  ABOUT 

30  ml. 

ep.  Epiglottis;  fc,  foramen  cecum; 
t^  and  f*  median  and  lateral  portions 
of  the  tongue. — (H»5.) 


TUB   SAUVABY  GLANDS 


•'•«>« 


tually  fuse  together,  but  the  groove  which  originally  separated  them 
remains  more  or  less  clearly  distinguishable  (Fig.  175),  the  vallate 
papilUe  (see  p.  430)  developing  immedia>    y  anterior  to  it. 

rnJ^n**  ™T'i''  T""""''  1  «"»<='''"■  '"8«n,  being  formed  of  a  central 
S^h^nT^^'^^r*'  T'°?^  "  *^'  »''*"  and  dorsally  by  mucous 
membrane  derived  from  the  floor  of  the  mouth  and  pharvnx     The 

Sn^lYnnT'  ""'f  P"!'^  °',«'^"  ''™"'»."'  the  .uTs'7n^ofTS: 
™Tl!.  ?K  r  T«  ^'  "•■•  'r*^"'"'  »"•*  »'"  of  «  »"">»>«"  of  extrinsic 
*fc>Hi.  Tlie  last  wo  muscles  are  innervated  by  the  vams  nerve  and 
^e  femaimng  extrinsic  muscles  receive  fibers  from  the  hyp%lo^  Xe 
fv  .^'SlV,r''''L'M'"";'^^''  '^'=  hypoglossal  and  parti*  a^'Vently! 
nL  in  fh^  T^^  ^^  ''"?'5'  !>""P""-  That  the  facial  should  take 
^t.nt  ?^^  "k^'I'V"?'"  ^,  "P«^"'' '™«  "«'  -node  of  develop! 
rTl  •  °T''  ''"'  "'!'  hypoglossal  has  been  seen  to  correspond  to 
certain  pnmanly  postcranial  metameres  (p.  .69),  and  its  reliUon  o 
smtcturestalungpartinthetormaUonofano^ganbiiongingotS^^^^ 
part  of  the  pharynx  seems  somewhat  anomalous.  It  may  be  suDDosed 
that  m  the  evolution  o  the  tongue  the  extrinsic  muscles.  togeSier^rtTa 
certain  amount  of  the  UnguaUs,  have  grown  into  the  tongu!  .hickS 

Such  an  invasion  of  the  tongue  by  mu.scles  from  posteri.-r  seicments 
would  explam  the  distribution  of  its  sensory  nerves  (Fig    176)     The 

fromThi^jTr'  "i*"" '"  •r*'""'  """'"^  ""'"""y  ^  '"PP'i*'l  by  branches 
^Sl  .^t""*  rl?f  T"""'  "^^  ^'  Posterior^rtio/ might  be 
^ted  to  be  suppbed  by  tiie  sevenUi.  There  seems,  however,  to  have 
b«n  a  dislocation  forward,  if  ,t  may  be  so  expressed,  of  the  mucous  mem- 
brane^ tiie  sensoor  distribution  of  tiie  nintii  nerve  extending  forward  upon 
the  postenor  part  of  the  anterior  portion  of  die  tongue,  while  a  consider- 
Th!  H-T"vf  H°'  '^'posterior  portion  is  suppUed  by  tiie  tenA  ner^e. 
Jntfr^t  ,  »f  °°  °'  •*'  "°^'y  ^^"  °'  *«  facial  is  probably  confined 
de^mV  ,?'  '""".T  T^°°'  'h°"«''  f"fther  informktion  is  needed  to 
detoimine  the  exact  distribution  of  boUi  the  motor  and  sensory  fibera  of 
this  nerve  in  die  tongue.  "cuaory  noers  01 

The  Development  of  the  Salivary  Glands.-In  embryos  of 
about  8  mm.  a  slight  furrow  may  be  observed  in  the  floor  of  the 
groove  which  connects  the  lip  grooves  of  the  upper  and  lower  jaws 
at  the  angle  of  the  mouth  and  may  be  known  as  the  cheek  groove. 
In  later  stages  this  furrow  deepens  and  eventually  becomes  closed 
m  to  form  a  hollow  tubular  structure,  which  in  embryos  of  17  mm. 


agi 


mc  MUVAKY  0L4MDS 


has  leparated  from  the  epithelium  of  the  floor  of  the  cheek  groove 
pt  at  its  anterior  end  and  has  become  embedded  in  the  connec- 
tive tissue  of  the  cheek.  This  tube  is  readily  recognizable  as  the 
parotid  gland  and  duct,  and  from  the  latter  as  it  passes  across  the 
masseter  muscle  a  pouch-like  outgrowth  is  early  formed  which  prob- 
ably represents  the  socio  parotidis. 


FlO.  «76. — D«0IIA1I  OF  THE  DlSTMBUnON  OF  IHE  SeNSOKY  NEitVES  OF  THE  TONGVI. 

The  area  supplied  by  the  fifth  (and  seventh)  nerve  is  indicated  by  the  transverse 
lines-  that  of  tl  -^  ninth  by  the  oblique  lines;  and  that  of  the  tenth  by  the  small  circles. 
— (Zmnfer.) 

The  submaxillary  gland  and  duel  appear  in  embryos  of  about 
13  mm.  as  a  longitudinal  ridge-like  thickening  of  the  epithelium 
of  the  floor  of  the  alveolo-lingual  groove  (see  p.  190).    This  ridge 


UUVAKY  OLAMDI 


•93 


gniduaUy  Mp«,«  from  behind  forward  from  the  floor  of  the 
groove  «.d  Ink.  into  the  .ubjaccnt  connective  ,i„ue,  retliiV 

wWch  indicate,  the  po.ition  of  the  opening  of  the  duct.    In  the 
SIS'  1  '^  "";!  ""P*"'  '"  '='"''^y°»  °'  '4.4  mm.  five  .md 
Lctt'cZTr-"'  '"= 'P'">^'"""  (^*  '77,5i).  which  iTte 
^r,!^f    ?    7  ^  !"  '"""'^'  "'  '"'"  "  '"  *'■"•  ""d  constitute  a 
joup  of  gland,  which  are  generally  spolcen  of  as  the  sublingual 

A.  these  representaUve.  of  the  various  glands  increase  in  length, 


A£m. 


5.*f«,  submirillary  du"  ri  "S^J.      ""''^''  '''''  ""ylol-yoidiM,  sublingu.l  gltnd; 

they  become  lobed  at  their  deeper  ends,  and  the  lobes  later  give 
nse  to  secondary  outgrowths  which  branch  repeatedly  the  tLfnll 

sdH  for,^    duct  portions  of  the  structures,  the  alveoli  remLng 
sohd  for  a  longer  time,  although  they  eventually  also  become  hollow 

pri^'r^°out™la**'dl1'l,''''7''''  "^^  '"bmaxillary  consist,  of  a  single 
oi  onginauy  separate  parts.    The  sublingual  glands  of  adult 


394 


THE    PBAKYNX 


anatomy  are  usuaUy  described  as  of^^^  T"  ^^  ^"^'ft^h^tti^ 
•  mmilir  of  separate  ducts.  This  arises  from  the  fact  that  «>e  ma^"7 
of  r  glandsTwch  form  in  the  vicinity  of  the  opening  of  Wharton, 
duct  reS  quite  small,  only  one  of  them  on  each  side  givmg  nse  to  &e 

of  sS  glands,  but  from  fourteen  to  sixteen  pairs  there  bemg  usuaUy 
from  eleven  to  thirteen  alveolo-Ungual  glands  on  each  side. 

The  Development  of  the  Pharynx.-The  pharynx  represents 
the  most  anterior  part  of  the  archenteron,  that  portion  in  which  the 
branchial  arches  develop,  and  in  tile  embryo  it  is  relatively  much 
longer  than  in  the  adult,  the  diminution  being  brought  about  by 
the  folding  in  of  the  posterior  arches  and  the  formation  of  the  sinus 
pnecervicalis   already   described    (p.   97)-    Between   the   various 
branchial    arches,   grooves   occur,   representing   the   endodermal 
portions  of  the  grooves  which  separate  the  arches.    During  develop- 
ment the  first  of  these  becomes  converted  into  the  tympanic  cavity 
of  the  ear  and  the  Eustachian  tube  (see  Chapter  XV);  the  second 
disappears  in  its  upper  part,  the  lower  persisting  as  the  fossa  in 
which  the  tonsU  is  situated;  while  the  lower  parts  of  the  remaining 
two  are  represented  by  the  sinus  piriformis  of  the  larynx  (His),  and 
also  leave  traces  of  their  existence  in  detached  portions  of  their  epi- 
thelium which  form  what  are  termed  the  branchial  epilheltal  bodies, 
and  take  part  in  the  formation  ol  the  thyreoid  and  thymus  glands. 
In  the  floor  of  the  pharynx  behind  the  thickenings  which  pro- 
duce the  tongue  there  is  to  be  found  in  early  stages  a  pair  of  thick- 
enings passing  horizontally  backward  and  uniting  in  front  so  that 
they  res.  mble  an  inverted  U  (Fig.  178,  /).    These  ridges,  which 
form  what  is  termed  the  furcula  (His),  are  concerned  in  the  forma- 
tion of  parts  of  the  larynx  (see  p.  334).    I"  the  part  of  the  roof  of 
the  pharynx  which  comes  to  lie  between  the  opemngs  of  the  Eusta- 
chian tubes,  a  collection  cf  lymphatic  tissue  takes  place  beneath 
the  mucous  membrane,  forming  the  pharyngeal  tonsil,  and  imme- 
diately behind  this  there  is  formed  in  the  median  hne  an  upward  y 
projecting  pouch,  the  pharyngeal  bursa,  first  certainly  noUceable 
in  embryos  6.5  mm.  in  length. 


THE  BKANCBIAL  EPITHELIAL  BODIES 


»95 


This  bursa  has  very  generally  been  regarded  as  the  persistent  remains 
of  Rathke's  pouch  (p.  285),  especially  since  it  is  much  more  pronounced 
in  fetal  than  in  adult  life.  It  has  been  shown,  however,  that  it  is  formed 
quite  independently  of  and  posterior  to  the  true  Rathke's  pouch  (Killian), 
though  what  its  significance  may  be  is  still  uncertain. 

The  lonsils  are  formed  from  the  epithelium  of  the  second  bran- 
chial groove.  At  about  the  fourth  month  solid  buds  begin  to  grow 
from  the  epithelium  into  the  subjacent  mesenchyme,  and  depressions 
appear  on  the  surface  of  this  region.  Later  the  buds  become  hollow 
by  a  comification  of  their  central  cells,  and  open  upon  the  floor  of 
the  depressions  which  represent  the 
crypts  of  the  tonsil.  In  the  meantime 
lymphocytes,  concerning  whose  origin 
there  is  a  difference  of  opinion,  collect  in 
the  subjacent  mesenchyme  and  eventu- 
ally aggregate  to  form  lymphatic  follicles 
in  close  relation  with  the  buds.  Whether 
the  lymphocytes  wander  out  from  the 
blood  into  the  mesenchyme  or  are  derived 
directly  from  the  epithelium  or  the  mes- 
enchyme cells  is  the  question  at  issue.  ^'°-  178— Thi  Flooi  of 
—L  ^  .,  .  ^  .  THE  Pharynx  of  an  Embryo 
The  tonsil  may  grow  to  a  size  sufficient   of  3.15  mm. 

to  fill  up  completely  the  groove  in  which     /  Furcuia;  t,  tuberculum  im- 
it  forms,  but  not  infrequently  a  marked 

depression,  the  fossa  supratonsillaris,  exists  above  it  and  represents 
a  portion  of  the  original  second  branchial  furrow. 

The  groove  of  Rosmmiiller,  which  was  at  one  time  thought  to  be 
also  a  remnant  of  the  second  furrow,  is  a  secondary  depression 
which  appears  in  embryos  of  11,5  cm.  behind  the  opening  of  the 
Eustachian  tube,  in  about  the  region  of  the  third  branchial  furrow. 

The  Development  of  the  Branchial  Epithelial  Bodies.— These  are 
structures  which  arise  either  as  thickenings  or  as  outpouchings  of 
the  epithelium  lining  the  lower  portions  of  the  inner  branchial  fur- 
rows. Five  pairs  of  these  structures  are  developed  and,  in  addition, 
there  is  a  single  unpaired  median  body.  This  last  '.  ;akes  its  appear- 
ance in  embryos  of  about  3  mm.,  and  gives  rise  to  the  major  por- 


i- 
■Nl 


II 


39^ 


THE   BSANCHIAL  EPIIHELUL  BODIES 


tion  of  the  thyreoid  body.  It  is  situated  immediately  behind  the 
anterior  portion  of  the  tongue,  at  the  apex  of  the  groove  between 
this  and  the  posterior  portion,  and  is  first  a  slight  pouch -like  depres- 
sion. As  it  deepens,  its  extremity  becomes  bilobed,  and  after  the 
embryo  has  reached  a  length  of  6  mm.  it  becomes  completely  sepa- 
rated from  the  floor  of  the  pharynx.  The  point  of  its  original 
origin  is,  however,  permanently  marked  by  a  circular  depression, 
the  foramen  cacum  ^ig.  1 7  5,  /c) .  Later  the  bilobed  body  migrates 
down  the  neck  and  becomes  a  solid  transversely  elongated  mass 
(Fig.  179,  th),  into  the  substance  of  which  trabecute  of  connective 
tissue  extend,  dividing  it  into  a  network  of  anastomosing  cords  which 


Fro  170  — RECONST»ucnoNS  or  the  Bhanchul  Epithelial  Bodies  or  Eubkyos. 

■     '  OF  (A)   14  lal.  AND  (B)  26  MM. 

00,  Aorta;  Uk,  literal  thyreoid;  ph,  pharyia;  j«»'  and  ptV,  parathyreoids;  <»,  thyreoid; 
(Ay,  thymus;  vc,  vena  cava  superior. — {Tourneux  and  Verdun.) 

later  divide  transversely  to  form  follicles.  AVhen  the  embryo  has 
reached  a  length  of  2.6  cm.,  a  cylindrical  outgrowth  arises  from  the 
anterior  surface  of  the  mass,  usually  a  little  to  the  left  of  the  median 
line,  and  extends  up  the  neck  a  varying  distance,  forming,  when  it 
persists  until  adult  life,  the  so-called  pyramid  of  the  thyreoid  body. 

This  account  of  the  pyramid  follows  the  statements  made  by  recent 
workers  on  the  question  (Tourneux  and  Verdun);  His  has  claimed  that 
it  is  the  remains  of  the  stalk  connecting  the  thyreoid  with  the  floor  of  the 
pharynx,  and  which  he  terms  the  Ihyreo-glossal  duct. 

Two  other  pairs  of  bodies  enter  into  intimate  relations  with  the 


THE  BKAKCHIAL  EPITHEUAL  BODIES 


m 


Ikm  tV 


pthm  III 


thyreoid,  forming  what  have  been  termed  the  parathyreoid  bodies 
(Fig.  179,  plh^  and  pth').  One  of  these  pairs  arises  as  a  thickening 
of  the  dorsal  portion  of  the  fourth  branchial  groove  and  the  other 
comes  from  the  corresponding 
portion  of  the  third  f-oove. 
The  iiembers  of  the  former 
pair,  alter  separating  from  their  pikm  iv 
points  of  origin,  come  to  lie  on 
the  dorsal  surface  of  the  lateral 
portions  of  the  thyreoid  body 
(Fig.  180,  pthm  IV)  in  close 
proximity  to  the  lateral  thy- 
reoids, while  those  of  the  other 
pair,  passing  further  backward, 
come  to  rest  behind  the  lower 
border  of  the  thyreoid  (Fig.  180, 
pthm  III).  The  cells  of  these 
bodies  do  not  become  divided 
into  cords  by  the  ingrowth  of 
connective  tissue  to  the  same 
extent  as  those  of  the  thyreoids, 
nor  do  they  become  separated 
into  follicles,  so  that  the  bodies 
are  readily  distinguishable  by 
their  structure  from  the  thy- 
reoid. 

From  the  ventral  portion  of 
the  third  branchial  groove  a 
pair  of  evaginations  develop, 
similar  to  those  which  produce 
the  lateral  thyreoids.  These  elongate  greatly,  and  growing  down- 
ward ventrally  to  the  thyreoid  and  separating  from  their  points  of  ori- 
gin, come  to  lie  below  the  thyreoids,  forming  the  thymus  gland  (Fig. 
'79i  '*>)•  As  development  proceeds  they  pass  further  backward 
and  come  eventually  to  rest  upon  the  anterior  surface  of  the  peri- 


FiG.  180. — Thyreoid,  Tyhmus  a.nd 
Epithelial  Bodies  of  a  New-bosn 
Child. 

fthm  III  and  flhm  IV,  Paralhyreoiiis; 
sd,  thyreoid;  thm  III,  thymus;  Ihm  IV, 
lateral  thyreoid. — (Groschuff.) 


398 


IHB  BBANCHIAI.  XPRHEUAI.  BODIES 


cardium.  The  cavity  which  they  at  first  contain  is  early  obliterated 
and  the  glands  assume  a  lobed  appearance  and  become  traversed  by 
trabecule  of  connective  tissue.  Lymphocytes,  derived,  according 
to  some  recent  observations,  directly  from  the  epithelium  of  the 
glands,  make  their  appearance  and  gradually  increase  in  number 
until  the  original  epithelial  cells  are  represented  only  by  a  number 
of  peculiar  spherical  structures,  consisting  of  cells  arranged  in  con- 
centric layers  and  known  as  HassaWs  corpuscles. 

The  glands  increase  in  size  until  about  the  fifteenth  year,  after 


FlO.  181,— DUOXAH  SBOWINO  TBI  OKicm  Or  TBE  Vawous  Bxanchiai.  Efuhxual 

Bodies. 

Uk,  Lateral  thyreoids;  M  ultimobranchial  bodies;  pkf^  and  ^*/',  parathyieoids;  tk, 

median  thyreoid;  thy,  thymus;  I  w  IV,  branchial  grooves.— (AToiB.) 

which  they  gradually  undergo  degeneration  into  a  mass  of  fibrous 
and  adipose  tissue. 

A  pair  of  evaginations  very  similar  to  those  that  give  rise  to  the 
thymus  are  also  formed  from  the  ventral  portion  of  the  fourth 
branchial  groove  (Figs.  179,  A  and  181,  Ith).  As  a  rule  they  com- 
pletely disappear  in  later  stages  of  development,  but  occasionally 


IHE    asOTHAOVS 


299 


they  undergo  differentiation  into  small  masses  of  thymus-like  tissue, 
which  remain  associated  with  the  parathyreoids  from  the  same  arch 
(Fig.  i8o,  rtoi  IV).  They  have  been  termed  lateral  thyreoids,  but 
the  term  is  a  misnomer,  since  they  take  no  essential  part  in  the  for- 
mation of  the  thyreoid  body. 

Finally,  a  pair  of  outgrowths  arise  from  the  floor  of  the  pharynx 
just  behind  the  fifth  branchial  arch,  in  the  region  where  the  fifth 
groove,  if  developed,  would  occur.  These  ultimo-branchial  bodies, 
as  they  have  been  called,  usually  undergo  degeneration  at  an  early 
stage  and  disappear  completely,  though  occasionally  they  persist 
as  cystic  structures  embedded  in  the  substance  of  the  thyreoid. 

The  relation  of  these  various  structures  to  the  branchial  grooves  is 
shown  by  the  annexed  diagram  (Fig.  i8i),  and  from  it,  it  will  be  seen 
that  the  bodies  derived  from  the  third  and  fourth  grooves  are  serially 
equivalent.  Comparative  embryology  makes  this  fact  still  more  evident, 
since,  in  the  lower  vertebrates,  each  branchial  groove  contributes  to  the 
formation  of  the  thymus  gland.  The  terminology  used  above  for  the 
various  bodies  is  that  generally  applied  to  the  mammalian  organs,  but  it 
would  be  better,  for  the  sake  of  comparison  with  other  vertebrates,  to 
adopt  the  nomenclature  proposed  by  Groschuff,  who  terms  each  lateral 
thyreoid  a  thymus  IV,  while  each  thymus  lobe  is  a  thymus  III.  Similarly 
the  parathyreoids  are  termed  para^ymus  III  and  IV,  the  term  thyreoid 
being  limited  to  the  median  thyreoid. 

The  Musculature  of  tlte  Pharynx. — ^The  pharynx  differs  from 
other  portions  of  the  archenteron  in  the  fact  that  its  walls  are  fur- 
nished with  voluntary  muscles,  the  principal  of  which  are  the  con- 
strictors and  the  stylo-pharyngeus.  This  peculiarity  arises  from 
the  relations  of  the  pharynx  to  the  branchial  arches.  It  has  been 
seen  that  in  the  higher  man.  nalia  the  dorsal  ends  of  the  third, 
fourth,  and  fifth  branchial  cartilages  disappear;  the  muscles  origin- 
ally associated  with  these  structiu-es  persist,  however,  and  give  rise 
to  the  muscles  of  the  pharynx,  which  consequently  are  innervated 
by  the  ninth  and  tenth  nerves. 

The  DeTelopment  of  the  (Esophagus.— From  the  ventral 
side  of  the  lower  portion  of  the  pharynx  an  evagination  develops 
at  an  early  stage  which  is  destined  to  give  rise  to  the  organs  of 


J 
1 1 


300 


THX   STOMACH 


respiration;  the  development  of  this  may,  however,  be  couveniently 
postponed  to  a  later  chapter  (Chapter  XII). 

The  oesophagus  is  at  first  a  very  short  portion  of  the  jrchenteron 
(Fig.  i8«,  A),  but  as  the  heart  and  diaphragm  recede  into  the 
thorax,  it  elongates  (Fig.  182,  B)  until  it  eventually  forms  a  consider- 
able portion  of  the  digestive  tract.  Its  endodermal  lining,  like  that 
of  the  rest  of  the  digestive  tract  except  the  pharynx,  is  surrounded 


FlO.  183— RlCONSTRCCTIONS  OF  TB£  DiGISTIVE  TkACI  OF  EMBRYOS  OF  M)  4  a  IDJ 
AND  (B)  5  IOC. 

M,  Allantois;  rf  cloaca;  /,  lung;  li,  liver;  Rp,  Rathke's  pouch;5,  stomach;  (.  tongue;  M. 
thyreoid  body;  Wd,  Wolffian  duct;  y,  yolk-stalk.— (ifM.) 

by  splanchnic  mesoderm  whose  cells  become  converted  into  non- 
striated  muscular  tissue,  which,  by  the  fourth  month,  has  separated 
into  an  inner  circular  a".d  an  outer  longitudinal  layer. 

The  Development  of  the  Stomach  and  Intestines.—By  the 
time  the  embryo  has  reached  a  length  of  about  $  mm.  its  constriction 


THE  INIBSTim 


301 


from  the  yolk-sac  has  proceeded  so  far  that  a  portion  of  the  digestive 
tract  anterior  to  the  yolk-sac  can  be  recognized  as  the  stomach  and 
a  portion  posterior  as  the  intestine.  As  first  the  stomach  is  a  simple, 
spmdie-shaped  enlargement  (Fig.  182)  and  the  intestine  a  tube 
without  any  coils  or  bends,  but  since  in  later  stages  the  intestine 
grows  much  more  rapidly  in  length  than  the  abdominal  cavity,  a 
coiling  of  the  intestine  becomes  necessary. 

The  elongation  of  the  stomach  early  produces  changes  in  its 
position,  its  lower  end  bending  over  toward  the  right,  while  its  upper 
end,  owing  to  the  development  of  the  liver,  is  forced  somewhat 
toward  the  left.  At  the  same  time  the  entire  organ  undergoes  a 
rotation  about  its  longitudinal  axis  through  nearly  ninety  degrees, 
so  that,  as  the  result  of  the  combination  of  these  two  changes,  what 
was  originally  its  ventral  border  becomes  ite  lesser  curvature  and 
what  was  originally  its  left  surface  becomes  its  ventral  surface. 

Hence  it  is  that  the  left  vagus  nerve  passes  over  the  ventral  and 
the  right  over  the  dorsal  surface  of  the  stomach  in  the  adult. 

In  the  meantime  the  elongation  of  the  oesophagus  has  carried 
the  stomach  further  away  from  the  lower  end  of  the  pharynx,  and 
from  beng  spindle-shaped  it  has  become  more  pyriform,  as  in  the 
adult.  The  fundus,  it  may  be  noted,  is  not  due  to  a  general  en- 
largement of  the  organ  but  to  a  local  outpouching  of  the  upper 
dorsal  portion  of  its  wall. 

The  growth  of  the  intestine  results  in  its  being  thrown  into  a  loop 
opposite  the  point  where  the  yolk-stalk  is  still  connected  with  it, 
the  loop  projecting  ventrally  into  the  portion  of  the  coelomic  cavity 
which  is  contained  within  the  umbilical  cord,  and  being  placed  so 
that  Its  upper  limb  lies  to  the  right  of  the  lower  one.  Upon  the  latter 
a  slight  pouch-like  lateral  outgrowth  appears  which  is  the  beginning 
of  the  cacum  and  marks  the  line  of  union  of  the  future  small  and  large 
intestine.  The  small  intestine,  continuing  to  lengthen  more  rapidly 
than  the  laige,  assumes  a  sinuous  course  (Fig.  183),  in  which  it  is 
possible  to  recognize  six  primary  coils  which  continue  to  be  recog- 
nizable until  -idranccd  stages  of  development  and  even  in  the  adult 
(Mall).    The  first  of  these  is  at  first  indistinguishable  from  the 


f! 


30» 


THE  niTKSTOK 


pyloric  portion  of  the  stomach  and  can  be  recognized  as  the  duo- 
denum only  by  the  fact  that  it  has  connected  with  it  the  ducts  of  the 
liver  and  pancreas;  as  development  proceeds,  however,  its  caliber 
diminishes  and  it  assumes  the  appearance  of  a  portion  of  the 
intestine. 

The  remaining  coils  elongate  rapidly  and  are  thrown  into 
numerous  secondary  coils,  all  of  which  are  still  contained  within  the 


Fio.  183. — Reconstruction  of  Ehbryo  of  jo  mi. 

C,  Cecum;  K,  kidney;^,  liver; S,  storaach;5C,  suprarenal  bodies;  W,  mesonephras. — 

(Mutf.) 

coelom  of  the  umbilical  cord  (Fig.  184).  When  the  embryo  has 
reached  a  length  of  about  40  mm.  the  coils  rather  suddenly  return 
to  the  abdominal  cavity,  and  now  the  cscum  is  thrown  over  toward 
the  right,  so  that  it  comes  to  lie  immediately  beneath  the  liver  on  the 
right  side  of  the  abdominal  cavity,  a  position  which  it  retains  tmtil 
about  the  fourth  month  after  birth  (Treves).  The  portion  of  the 
large  ir  testine  which  formerly  projected  Into  the  umbilical  coelom  now 


THB  nnxsTiinc 


303 


lies  transversely  across  the  upper  part  of  the  abdomen,  crossing  in 
front  of  the  duodenum  and  having  the  remaining  portion  of  the  small 
intestine  below  it.  The  elongation  continuing,  the  secondary  coils 
of  the  small  intestine  become  more  numerous  and  the  lower  portion 
of  the  large  intestine  is  thrown  into  a  loop  which  extends  trans- 
versely across  the  lower  part  of  the  abdominal  cavity  and  represents 
the  sigmoid  flexure  of  the  colon.  At  the  time  of  birth  this  portion 
of  the  large  intestine  is  relatively  much  longer  than  in  the  adult, 
amounting  to  nearly  half  the  entire  length  of  the  colon  (Treves), 
but  after  the  fourth  month  after  birth  a  readjustment  of  the  relative 


Tio.  184. — Reco-wudction  of  thi  Intzstihz  of  .n  Ehbiyo  of  iq  lof.    The 
Fiousis  OM  TBI  Intistine  Imdicati  the  PaiHAiY  Coos.— (Matt.) 

lengths  of  the  parts  of  the  colon  occurs,  the  sigmoid  flexure  becoming 
shorter  and  the  rest  of  the  colon  proportionally  longer,  whereby  the 
caecum  is  pushed  downward  until  it  lies  in  the  right  iliac  fossa,  the 
ascending  colon  being  thus  established. 

When  this  condition  has  been  reached,  the  duodenum,  after 
passing  downward  for  a  short  distance  so  as  to  pass  dorsally  to  the 
transverse  colon,  bends  toward  the  left  and  the  secondary  coils 
derived  from  the  second  and  third  primary  coils  come  to  occupy 
the  left  upper  portion  of  the  abdominal  cavity.  Those  from  the 
fourth  primary  coil  pass  across  the  middle  line  and  occupy  the  right 


m 

m 
w 

m 


304 


IBX  INRSTINX 


upper  part  of  the  abdomen,  those  from  the  fifth  cross  back  <«ain  to 
the  left  lumbar  and  iliac  regions,  and  those  of  the  sixth  take  pos- 
session of  the  false  pelvis  and  the  right  iliac  region  (Fig.  185). 

SUght  variations  from  this  arrangement  are  not  «»''«!,«■''- .^'j* 
occurs  with  sufficient  frequency  to  be  regarded  as  the  normiQ.    A  faUure 


FIO.    185.-REPRESEKTAT.ON    OF   THE   C0ILP.GS    °\^^l^^^^fuT)    *""" 

Condition.    The  Numbers  indicate  the  Primary  Con^.-IM"*) 

in  the  readjustment  of  the  relative  lengths  of  the  ^i^^f"' I!"J',,°[„'|'^ 
colon  may  also  occRsionaUy  occur,  in  which  case  the  cxcum  wiU  retam  its 
embryonic  position  beneath  the  liver. 

The  yolk-stalk  is  continuous  with  the  intestine  at  the  extremity 
of  the  loop  which  extends  out  into  the  umbilical  cnelom,  and  when  the 


THE  INTESTINE 


30s 


pnmaiy  coils  become  apparent  its  point  of  attachment  lies  in  the 
region  of  the  sixth  coil.  As  a  rule,  the  caliber  of  the  stalk  does  not 
increase  proportionally  with  that  of  the  intestine,  and  eventually 
its  embryomic  portion  disappears  completely.  Occasionally,  how- 
ever, this  portion  of  it  does  partake  of  the  increase  in  size  which 
occurs  in  the  intcstin?!,  and  it  forms  a  blind  pouch  of  varying  length, 
known  as  Meckel's  diverticulum  (see  p.  113). 

The  cacum  has  been  seen  to  arise  as  a  lateral  outgrowth  at  a 
time  when  the  intesUne  is  first  drawn  out  into  the  umbilicus.    During 
subsequent  development  it  continues  to  in- 
crease in  size  until  it  forms  a  conical  pouch 
arising  from  the  colon  just  where  it  is  joined 
by  the  small  intestine  (Fig.  186).    The  en- 
largement of  its  terminal  portion  docs  not  keep 
pace,  however,  with  that  of  the  portion  near- 
est the  intestine,  but  it  becomes  gradually 
more  and  more  marked  off  from  it  by  its  lesser 
caliber  and  gives  rise  to  the  vermiform  ap- 
pendix.   At  birth  the  original  conical   form 
of  the  entire  outgrowth  is  still  quite  evident, 
though  it  is  more  properly  described  as  funnel- 
shaped,  but  later  the  proximal  part,  continuing  to  increase  in  diam- 
eter at  the  same  rate  as  the  colon,  becomes  sharply  separated  from 
the  appendix,  forming  the  csecum  of  adult  anatomy. 

Up  to  the  time  when  the  embryo  has  reached  a  length  of  14  mm., 
the  inner  surface  of  the  intestine  is  quite  smooth,  but  when  a  length 
of  19  mm.  has  been  reached,  the  mucous  membrane  of  the  upper 
portion  becomes  thrown  into  longitudinal  folds,  and  later  these  make 
their  appearance  throughout  its  entire  length  (Fig.  187).  Later,  in 
embryos  of  60  mm.,  these  folds  break  up  into  numbers  of  conical 
processes,  the  ,lli,  which  increase  in  number  with  the  development 
of  the  mtestine,  the  new  vUli  appearing  in  the  intervals  between  those 
already  present.  Villi  are  formed  as  well  in  the  large  as  in  the  small 
mtestine,  but  in  the  former  they  decrease  in  size  as  development 
proceeds  and  practically  disappear  toward  the  end  of  fetal  life. 


Fio.  i86.— Cjtcuii  or 
I.MBRYO  or  lo.a  CM. 
c,  Colon;  i,  ileum. 


e'.j 

iifl 


jo6  iB>  uvu 

In  the  early  itacet  the  endodermal  lining  of  the  digestive  tract  aMumet 
a  considerable  thicltneM,  the  lumen  of  -he  ««>(!'»?)'•»"<•  "PP'l/?.ni^. 
the  .mall  intestine  being  reduced  to  a  very  .mall  caliber.  In  Uter  .tage. 
a  rapid  increaM  in  the  sixe  of.  the  lumen  occur.,  apparently  ^^^ 
with  the  formaUon  of  caviUes  or  vacuole,  in  the  endodermal  epitheUum. 
TheM  increaM  in  si«,  the  neighboring  cell,  arrange  themMlve.  in  an 
epithelial  layer  ar..und  their  wall,  and  they  eventually  break  through  into 
the  general  lumen.  They  are  «.metime.  .ufficicntly  large  to  give  the 
appearance  ui  diverUcula  of  the  gut,  but  later  they  flatten  out,  their 
cavitie.  becoming  portion,  of  the  general  lumen.  

In  the  ca«  of  the  duodenum  the  thickening  of  the  endodermal 
lining  proceed,  to  .uch  an  extent  that  in  embiyos  of  •™™,";5  mm.  to 
14.5  mm.  the  lumen  i.  completely  obhterated  immediately  below  the 
opening  of  the  hepatic  and  pancreatic  ducts.  Thi.  condition  1.  interesting 
inconnection  with  the  occasional  occurrence  in  new-born  children  of  an 
atresia  of  the  duodenum.  Under  normal  condiUon.,  however,  the  lumen 
i.  restored  by  the  proce..  of  vacuolization  de«:ribed  above. 


ini   sHOwrao  the  Lonoitudinai.  Folds  reoii  which  thi  Viiii  am  Fowod. 
(.Btrry.) 

The  Development  of  the  Liver.— The  liver  makes  its  appear 
ance  in  embryos  of  about  3  mm.  as  a  longitudinal  groove  upon  tht 
ventral  surface  of  the  archenteron  just  below  the  stomach  anc- 
between  it  and  the  umbilicus.  The  endodermal  cells  lining  th. 
anterior  portion  of  the  groove  early  undergo  a  rapid  proliferaUor, 
and  form  a  solid  mass  which  projects  ventrally  into  the  substance 


TEC  UVBt 


307 


of «  horiionul  shelf,  the  septum  transveraum  (see  p.  318),  attached 
to  the  ventral  wall  of  the  body.  This  solid  mass  (Fig.  188,  I) 
forms  the  beginning  of  the  liver  proper,  while  the  lower  portion  of 
the  groove,  which  remains  hollow,  represents  the  future  gall-bladder 
(Fig.  :88,  B).  Constrictions  appearing  between  the  intestine  and 
both  the  hepauc  and  cysUc  portions  of  the  organ  gradually  separate 
these  from  the  intestine,  until  they  are  united  to  it  only  by  a  stalk 
which  represents  the  ductus  chokdochus  (Fig.  188). 

The  further  development  of  the  liver,  so  far  as  its  external 


a,  GaU-bUdder;  d,  duodenum;  DV.  ductus  venosiu'  T   li«.r.  *  j„_  i 

antral  pane™.;  rL,  right  tab.  orth^'r^TsfitolS^h'-t^^L^"'"^  '"' 

form  is  concerned,  consists  in  the  rapid  enlargement  of  the  hepatic 
portion  until  It  occupies  the  greater  part  of  the  upper  half  of  the 
abdominal  cavity,  its  ventral  edge  extending  as  far  down  as  the 
umbilicus.  In  the  rabbit  its  substance  becomes  divided  into  four 
obes  corresponding  to  the  four  veins,  umbilical  and  vitelline,  which 
»Ifh'"Vu  Tu  **'^  '"""'  condition  occurs  in  ihe  human  embryo, 
although  the  lobes  are  not  so  clearly  indicated  upon  the  surface  as  in 
tne  rabbit.    The  two  vitelline  lobes  are  in  close  apposition  and  may 


I'll 


1'  I 


-08  THE   LIVE* 

almost  be  regarded  a^  one,  a  median  ventral  lobe  which  embraces 
the  ductus  venosus  (Fig.  i88,  B,  DV),  while  the  "-nbihcal  lobes  are 
more  lateral  and  dorsal  and  represent  the  right  (rl)  and  left  lobas 
of  the  adult  liver.  The  remaining  definite  lobes,  the  caudate 
(Spigelian)  and  quadrate,  are  of  later  formation,  standing  m  relation 
to  the  vessels  which  cross  the  lower  surface  of  the  hver. 

The  ductus  choledochus  is  at  first  wide  and  short,  and  near  its 
proximal  end  gives  rise  to  a  small  outgrowth  on  each  side,  one  of 
which  becomes  the  ventral  pancreas  (Fig.  i88,  B,  pm)  Later  the 
duct  elongates  and  becomes  more  slender,  and  the  gall-bladder  is 


F.O     I89.-TKANSVESSE    SECTION   THJOUOH   THE   LWER   O.   AN    EHB«0    OF  Fot» 

riu.  ioy  Months. 

,»,  Intestine;  I,  liver;  W,  Wolffian  bod)r.-(roW  »,«J  Z<^kerkandl) 

constricted  off  from  it,  the  connecting  stalk  beco^r^ing  the  cyftic 
duct.  The  hepatic  ducts  are  apparently  developer,  fron.  the  liver 
substance  and  are  relatively  late  in  appearing. 

Shortly  after  the  hepatic  portion  has  been  differentiated  its  sub- 
stance becomes  permeated  by  numerous  blood-vessels  (sinusoids) 
and  so  divided  into  anastomosing  irabecute  (Fig.  189).  These  arf 
at  first  irregular  in  size  and  shape,  but  later  they  become  more  slender 
and  more  regularly  cylindrical,  forming  what  have  been  termed  the 


THE  UVXK 


309 


hepatic  cylinders.  i  the  center  jf  each  cylinder,  where  the  cells 
which  form  it  meet  lu^ether.  a  fi  le  canal  appears,  the  beginning  of 
a  bile  capillary,  the  cylinders  tlius  becoming  converted  into  tubes 
with  fine  lumina.  This  occurs  at  about  the  fourth  week  of  develop- 
ment and  at  this  time  a  cross-section  of  a  cylinder  shows  it  to  be 
composed  of  about  three  or  four  hepatic  cells  (Fig.  190,  A),  among 
which  are  to  be  seen  groups  of  smaller  cells  («)  which  are  erythro- 
cytes, the  liver  having  assumed  by  this  time  its  haematopoietic  func- 
tion (see  p.  225).     This  condition  of  affairs  persists  until  birth,  but 


Fio.  190.— Transvekse  Sections  op  Portions  of  the  Liver  of  (/I)  a  Fetus  of  Six 

Months  AND  (B)  A  Child  of  Four  Years. 

be,  Bile  capillary;  e,  erythrocyte;  he,  hepatic  cylinder.— (ro/ift  and  Zuckeriandl.) 

later  the  cylinders  undergo  an  elongation,  the  cells  of  which  they  are 
composed  slipping  over  one  another  apparently,  so  that  the  cylin- 
ders become  thinner  as  well  as  longer  and  show  for  the  most  part 
only  two  cells  in  a  transverse  section  (Fig.  190,  B);  and  in  still  later 
periods  the  two  cells,  instead  of  lying  opposite  one  another,  may 
alternate,  so  that  the  cylinders  become  even  more  slender. 

The  bile  capillaries  seem  to  make  their  appearance  first  in  cylin- 
ders which  lie  in  close  relation  to  branches  of  the  portal  vein  (Fig.  191), 


I! 


310 


THE   UVEK 


and  thence  extend  throughout  the  neighboring  cylinders,  anastomos- 
ing with  capillaries  developing  in  relation  to  neighboring  portal 
branches.  As  the  extension  so  proceeds  the  older  capillaries  con- 
tinue to  enlarge  and  later  become  transformed  into  bile-ducts  (Fig. 
191,  C),  the  cells  of  the  cylinders  in  which  these  capillaries  were 
situated  becoming  converted  into  the  epithelial  lining  of  the  ducts. 

The  lobules,  which  form  so  characteristic  a  feature  of  the  adult 
liver,  are  late  in  appearing,  not  being  fully  developed  until  some 
time  after  birth.  They  depend  upon  the  relative  arrangement  of 
the  branches  of  the  portal  and  hepatic  veins;  these  at  first  occupy 
distinct  territories  of  the  liver  substance,  being  separated  from  one 
another  by  practically  the  entire  thickness  of  the  liver,  although  of 


Fio    lOI  — iNreCIID  BllE  CAPnX»MIS  OF  PlO  EMBKYOS  or  (.A)  8  CM.,  (B)  16  CK.,  AKD 

rro.  19'-        J  j^  Q^  f^^^  Pto.—iHendriclutm.) 

course  connected  by  the  sinusoidal  capillaries  which  lie  between  the 
hepatic  cylinders.  During  development  the  two  sets  of  branches 
extend  more  deeply  into  the  liver  substance,  each  invading  the 
territory  of  the  other,  but  they  can  readily  be  distinguished  from  one 
another  by  the  fact  that  the  portal  branches  are  enclosed  within  a 
sheath  of  connective  tissue  (Glisson's  capsule)  which  is  lacking  to 
the  hepatic  vessels.  At  about  the  time  of  birth  the  branches  of  the 
hepatic  veins  give  off  at  intervals  bunches  of  terminal  vessels,  around 
which  branches  of  the  portal  vein  arrange  themselves,  the  liver  tissue 
becoming  divided  up  into  a  number  of  areas  which  may  be  termed 


THE   PANCXEAS 


3" 


hepatic  islands,  each  of  which  is  surrounded  by  a  number  of  portal 
branches  and  contains  numerous  dichotomously  branching  hepatic 
terminals.  Later  the  portal  branches  sink  into  the  substance  of  the 
islands,  which  thus  become  lobed,  and  finally  the  sinking  in  extends 
so  far  that  the  original  island  becomes  separated  into  a  number  of 
smaller  areas  or  lobules,  each  containing,  as  a  rv  :,  a  single  hepatic 
terminal  (the  intralobular  vein)  and  being  surrounded  by  a  number 
of  portal  terminals  (interlobular  veins),  the  two  systems  being  united 
by  the  capillaries  which  separate  the  cylinders  contained  within  the 
area.  The  lobules  are  at  first  very  small,  but  later  they  increase  in 
size  by  the  extension  of  the  hepatic  cylinders. 

Frequently  in  the  human  liver  lobules  are  to  be  found  containing  two 
intralobular  veins,  a  condition  with  results  from  an  imperfect  subdivision 
of  a  lol>e  of  the  original  hepatic  island. 

The  liver  early  assumes  a  relatively  large  size,  its  weight  at  one 
time  being  equal  to  that  of  the  rest  of  the  body,  and  though  in  later 
embryonic  stages  its  relative  size  diminishes,  yet  at  birth  it  is  still  a 
voluminous  organ,  occupying  the  greater  portion  of  the  upper  half  of 
the  abdominal  cavity  and  extending  far  over  into  the  left  hypo- 
chondrium.  Just  after  birth  there  is,  however,  a  cessation  of 
growth,  and  the  subsequent  increase  proceeds  at  a  much  slower  rate 
than  that  of  the  rest  of  the  body,  so  that  its  relative  size  bcomes 
still  more  diminished  (see  Chap.  XVII).  The  cessation  of  growth 
affects  principally  the  left  lobe  and  is  accompanied  by  an  actual 
degeneration  of  portions  of  the  liver  tissue,  the  cells  disappearing 
completely,  while  the  ducts  and  blood-vessels  originally  present 
persist,  the  former  constituting  the  vasa  aberraniia  of  adult  anatomy. 
These  are  usually  especially  noticeable  at  the  left  edge  of  the  liver, 
between  the  folds  of  the  left  lateial  ligament,  but  they  may  also  be 
found  along  the  line  of  the  vena  cava,  around  the  gall-bladder,  and 
in  the  region  of  the  left  longitudinal  fissure. 

The  Development  of  the  Pancreas.— The  pancreas  arises  a 
little  later  than  the  liver,  as  two  or  three  separate  outgrowths,  one 
from  the  dorsal  surface  of  the  duodenum  (Fig.  192,  D  P)  usually  a 
little  above  the  liver  outgrowth,  and  one  or  two  from  the  lower  part 


i 


1-) 


3" 


THE  PANCBXAS 


\i 

i! 


of  the  common  bUe-duct.  Of  the  latter  outgrowths,  that  upon  the 
left  side  (Vps)  may  be  wanting  and,  if  formed,  early  disappears, 
while  that  of  the  right  side  {VpS)  continues  its  development  to  form 
what  has  been  termed  the  ventral  pancreas.  Both  this  and  the 
dorsal  pancreas  continue  to  elongate,  the  latter  lying  to  thi  left  of 

the  portal  vein,  while  the  former, 
at  first  situated  to  the  right  of 
the  vein,  later  grows  across  its 
ventral  surface  so  as  to  come  into 
contact  with  the  dorsal  gland, 
with  which  it  fuses  so  intimately 
that  no  separation  line  can  be 
distinguished.  The  body  and 
tail  of  the  adult  pancreas  rep- 
resent the  original  dorsal  out- 
growth, while  the  right  ventral 
pancreas  becomes  the  head. 

Both  the  dorsal  and  ventral 
outgrowths  early  become  lobed, 
and  the  lobes  becoming  second- 
arily lobed  and  this  lobation  re- 
peating itself  several  times,  the 
compound  tubular  structure  of 
►he  adult  gland  is  acquired,  the 
very  numerous  terminal  lobules 
becoming  the  secreting  acini, 
wliile  the  remaining  portions 
become  the  ducts.  Of  the  prin- 
cipal ducts,  there  are  at  first  two; 
that  of  th(.  uorial  pancreas,  the 
duct  of  Santorini,  opens  into  the  duodenum  on  'its  dorsal  surface, 
while  that  of  the  ventral  outgrowth,  the  duct  of  Wirsung,  opens 
into  the  ductus  choledochus.  When  the  fusion  of  the  two  portions 
of  the  gland  occurs,  an  anastomosis  of  branches  of  the  two  ducts 
develops  and  the  proximal  portion  of  the  duct  of  Santorini  may 


Fig.  IQ2. — Reconstruction  of  the 
Pancreatic  Odtgrowths  or  an  Embryo 
or  7.5  MM. 

D  Duodenum;  Dc,  ductus  conunuuis 
choledochus;  DP,  dorsal  pancreas;  Vpd, 
and  Vps,  right  and  left  ventral  pancreas. 
-(HeB)>.) 


UTESATintE  313 

degenerate,  so  that  the  secretion  of  the  entire  gland  empties  into 
the  common  bile-duct  through  the  duct  of  Wirsung. 

In  the  connective  tissue  which  separates  the  lobules  of  the  gland, 
groups  of  cells  occur,  which  have  no  connection  with  the  ducts  of 
the  gland,  and  form  what  are  termed  the  areas  ofLangerhans.  They 
arise  by  a  differentiation  of  the  cells  which  form  the  original  pancre- 
atic outgroTvths,  and  have  been  distinguished  in  the  dorsal  pancreas 
of  the  guinea-pig  while  it  is  still  a  solid  outgrowth.  They  gradually 
separate  from  the  remaining  cells  of  the  outgrowth  and  come  to  lie 
in  the  mesenchyme  of  the  gland  in  groups  into  which,  finally,  blood- 
vessels penetrate. 

LITERATURE. 


E.  T.  Bell;  '-The  Development  of  the  Thymus,"  Amer.  Joum.  of  Anat.^  v,  igo6. 
J.  M.  Berry;  "On  the  Development  of  the  Villi  of  the  Human  Intestine,"  Anat. 

Anxeiger,  xvi,  1900. 
L.  BOLK;  "Die  Entwicklungsgeschlchte  der  menschlichen  Lippcn,"  Anat.  Hejte. 

XT.IV,  1908. 
L.  Bout:  "Ueber  die  Gaumenentwicklung  tmd  die  Bedeutung  der  oberen   Zahn- 

leiste  beim  Menschen,"  Zeil.far  Morfhcl.  und  AtUhropol.,  xiv,  1911. 
J.  Bsachst:  "Recherches  sur  le  d^veloppcment  du  pancr^s  et  du  foie,''  Joum.  do 

fAnat.  ttitia  Physiol.,  xxxil,  1896. 
O.  C.  Bradley;  "A  Contribution  to  the  Morpholog)  and  Development  of  the  Mam- 
malian Liver,"  Joum.  Anat.  and  Physiol.,  XLin,  1908. 
H.  M.  DE  Bdslet;  "Die  ausseren  Formverhaltnisse  der  Leber  beim  menschlichen 

Embryo,"  Morphol.  Jahrb.,  XLn,  1910. 
R.  V.  Chahberlin:  "On  the  Mode  of  Disappearance  of  the  Villi  fnm  the  Colon  of 

Mamrrials,"  Anat.  Pxord.  ill,  1909. 
J.  H.  Chzevitz:  "Britrage  zur  Entwicklungsgeschlchte  der  Speicheldrtisen,"  Archvv 

far  Anal,  und  Physiol.,Anal.  Ablh.,  1885. 
H.  Fox:  "The  Pharyngeal  Pouches  and  Their  Derivatives  in  the  Mammalia,"  Anier. 

Joum.  Anat.,  vm,  X90S. 
K.  Grosgruff:  "  Ueber  das  Vorkommen  eines  Thymussegmentes  der  vierten  Kiemen- 

tasche  beim  Menschen,"  Anat.  Anzeiger,  xvil,  1900. 
O.  Grosser:  "Zut  Kenntnis  des  uitimobranchialen  Korpers  beim  Menschen,"  Anat. 

Anuiger,  xxxvn,  1910. 
L.  GrHnwald;  "Ein  Bcitzag  nir  !£ntstehung  tmd  Bedeutung  der  Gaumenmandeln," 

Anat.  Anuiger,  xxxvn,  19^0. 
J.  A.  HAlOkAR;  "Ei;.igc  Plattenmodelle  zur  Beleuchtung  der  frtiheren  embryonal 

Leberentwickluig,"  Arch.f.  Anat.  und  Phys.,  Anat.  Ablh.,  1893. 


J'i  I 


3M 


UIUATUKB 


1^1 


J.  A.  Haioux:  "  Notix  ttber  die  EntwicUnng  d«r  Zunge  und  der  MimdapdchddiflMn  , 

beim  Mauchen,"  Anat.  Atueiger,  xix,   1901. 
J.  A.  Hawuc  "  Studim  Hbcr  die  EntwicUiing  dn  Voiderdamu  und  einiger  angrcn- 

zender  Organe,"  Arch./,  mikrosk.  Anat.,  Lix  and  tx,  1903- 
K.  Hmxy:  "Zui  Entwickdungsgeicliichte  der  Pancteaianlagai  und  Duodenalpapillcn 

dea  Menschen,"  Arckiv/^  mikrosk.  Anat.,  LVl,  1900. 
K.  Hilly:  "Studien  aoer  Langerluunsche  Inseln,"  Arch,  far  mikrosk.  Anal.,l.xvn, 

1907. 
W.  F.  Hemdeickson:  "The  Development  of  the  Bile-capillariei  as  revealed  by  Golgi's 

Method,"  Johns  Hopkins  Hospital  BvUeiiH,  1898. 
W.  His:  "  Anatomie  menachlicher  Embiyoncn,"  Leipzig,  i88a-i886. 
F.  HocBSTimn:  "Ueber  die  Biidung  der  primiliven  Cboanen  beim  Menschen," 

Anat.  Angeiger,  vu,  :89a. 
N.  W.  INOALLS:  "A  Contribution  to  the  Embiyology  of  the  Liver  and  Vascular  System 

in  Man."  Anat.  Kicori,  n,  1908.  ' 

CM.  Jackson  : "  On  the  Development  and  Topography  of  the  Thoracic  and  Abdominal 

Viscera,"  Anai.  Record,  m,  1909. 
I .  P.  Johnson:  "The  Development  of  the  Mucous  Membrane  of  the  (Esophagus, 

Stomach  and  Small  Intestine  in  the  Human  Embiyo,"  Amer.  Jcum.  Anal.,  x, 

1910. 

E.  KALtire.  ■'  Bdtrilge  lur  Entwicklung  der  Zunge,  jte  Th.  Siugetiere.  I.  Sus  scrota," 

Anal.  Hefle,  xu,  1910. 

F.  Kiibil:  "Zur  Entwickelungsgeschichte  dss  menschlichen  Urogenital-apparatus," 

ArdmfV  Anal,  und  Pkysiat.,  Anal.  Aotk.,  1896. 

G.  Kiluan:  "Ueber  die  Bursa  und  Tonsilla  pharyngea,"  Marphol.  Jakrbuch,  xiv, 

1888. 
A.  Kohn:  "Die  EpithelkSrperchen,"  Ergebnisst  der  Anat.  und  ExlmcUungsgesch.,  a, 

1899. 
H.  KOSTZR:  "Zur  EntwicUungsgeschichte  der  Langethans'schen  Insebi  im  Pancreas 

beim  menschlichen  Embiyo,"  Arch.ftr  mikrosk.,  inat.,  LXIV,  1904. 
F.  T.  Lewis  and  F.  W.  Thvng:  "The  Regular  Occurrence  of  Intestinal  Diverticula 

in  Embryos  of  the  Pig,  Rabbit  and  Man,"  Amer.  Joum.  Anat.,  vn,  1908. 
F.  P.  Mall:  "Ueber  die  Entwickelung  des  menschlichen  Darmes  und  seiner  Lage 

beim  Erwachsenen,"  Archivfar  Anat.  und  Physiol.,  Anat.  AUh.,  Supplemtnl,  1897. 
F.  P.  Mall:  "A  Study  of  the  Structural  Unit  of  the  Liver,"  Amer.  Joum.  0/  Anal.,  v, 

1906. 
R.  May»:  "  Ueber  die  Biidung  des  Recessus  pharyngeus  medius  s.  Bursa  pharyngis  in 

zusammenhang  mit  der  Chorda  bei  menschlichen  Embryonen,"  Anol.  Anuiger, 

xxxvil,  1910. 
J.  F.  MxCKXL:  "Bildungsgeschichte  des  Daimkanals  der  Siiugethiere  imd  namentlich 

des  Menschen,"  Archivfar  Anat.  und  Physiol.,  iil,  1817. 
T.  MuoHIScn:  "  Ueber  die  Entwicklung  der  Langerhans'  schen   Inseln  bei  men- 
schlichen Embryonen,"  Arrh.fOr  mikrosk.  Anal.,  Lxxvl,  1911. 
W.  J.  Otis:  "  Die  Moiphogenese  und  Histogenese  des  AnalhOckers  nebst  Bemerkungen 

fiber  die  Entwicklung  der  Sphincter  ani  eitemus  beim  Menschen,"  Anat.  HefU, 

XXX,  1906. 


UXHATDUE 


3*5 


R.  M.  Pukce:  "The  Dcvelopmail  of  the  Islandi  of  Ungerhaiu  in  the  Himuui 

Embryo,"  Amtr.  Jtmm.  of  Anat,,  ii,  190a, 
C.  Kdn:  "  Ueber  Co  EntwicUung  der  Zithne  del  Menichen,"  Archiv  flir  miiroik. 

Awu.f  xxxviii,  1891. 

G.  Schom:  "ZurEntwickelungsgeschichte  det  secundlnn  Gtvmau,"  Anal.  Hifn 

XXXVI,  1908.  ' 

O.  ScHou:  "  Ueber  WoUsrachen  von  Sl»ndpunkt  der  Embryologie  und  patiiologischeo 

An»tomie,"  Arch.ftir  paiholoe-  AtuU.,  cxcvn,  1909. 
A.  Swain;  "Recherche*  lur  le  d^rebpement  du  foie,  du  tube  digestif,  de  I'arriire- 

cavit<  du  peritoine  et  da  m<ient»re,"  Journ.  it  tAnal.  a  de  la  PhysM.,  xxxil, 

1896,  and  xxxin,  1897. 
J.  Tandlei:  "Zur  Entwiclielungsgeschichte  des  menschUchen  Duodenum  in  frtlhen 

Embryonalstadicn,"  Marpkol.  Johrbuch,  xxix,  1900. 
P.  Thompson:  "a  Note  on  the  Development  of  the  Septum  Tiansvenum  and  the 

Liver,"  Joum.  Altai,  and  Phys.,  xui,  1908. 
F.  W.  Thyng:  "Models  of  the  Pancreas  in  Embiyos  of  the  Pig,  Rabbit,  Cat  and 

Man,"  Amer,  Jtmm.  Anal.,  vil,  1908. 
C.  TotDT  AND  E.  Zdckerxandl:  "Ueber  die  Form  und  Teiturverilnderungen  der 

menschUchen  Leber  wahrend  des  Wachsthums,"  SUtmgsber.  da  kais.  Altai. 

WissoKck.  Wiai.,  Ualk.-Nalurwus.  Claise,  ixxu,  1875. 
F.  Tonwiiux  and  P.  Vmdun:  "  Sur  les  premiers  d<veloppements  de  U  Thyroide,  du 

Thymus  et  des  glandes  parathyroidiennes  chez  I'homme,"  Joum.  it  TAnal.  el 

it  la  Physiol.,  xxxni,  1897. 
F.  Tkvis:  "Lectures  on  the  Anatomy  of  the  Intestinal  Canal  and  Peritoneum  in 

Man,"  Brilish  iteikal  Journal,  I,  1885. 


11 


CHAPTER  XI 

THE  DEVELOPMENT  OF  THE  PERICARDIUM,  THE 
PLEURO-PERITONEUM  Ain)  THE  DIAPHRAGM. 

It  has  been  seen  (p.  229)  that  the  heart  makes  its  appearance  at 
a  stage  when  the  greater  portion  of  the  ventral  surface  of  the  intes- 
tine is  still  open  to  the  yolk-sac.  The  ventral  mesoderm  splits  to 
form  the  somatic  and  splanchnic  layers  and  the  heart  develops  as  a 
fold  in  the  latter  on  each  side  of  the  median  line,  projecting  into  the 
coelomic  cavity  enclosed  by  the  two  layers  (Fig.  136,  A).  As  the 
constriction  of  the  anterior  part  of  the  embryo  proceeds  the  two 
heart  folds  are  brought  nearer  together  and  later  meet,  so  that  the 
heart  becomes  a  cylindrical  structure  lying  in  the  median  line  of  the 
body  and  is  suspended  in  the  coelom  by  a  ventral  band,  the  ventral 
mesocardium,  composed  of  two  layers  of  splanchnic  mesoderm 
which  extend  to  it  from  the  ventral  wall  of  the  body,  and  by  a 
similar  band,  the  dorsal  mesocardium,  which  unites  it  with  the 
splanchnic  mesoderm  surrounding  the  digestive  tract.  The  ven- 
tral mesocardium  soon  disappears  (Fig.  136  C)  and  the  dorsal  one 
also  vanishes  somewhat  later,  so  that  the  heart  comes  to  lie  freely 
in  the  coelomic  cavity,  except  for  the  connections  which  it  makes 
with  the  body-walls  by  the  vessels  which  enter  and  arise  from  it. 

The  coelomic  cavity  of  the  embryo  does  not  at  first  communicate 
with  the  extra-embryonic  coelom,  which  is  formed  at  a  very  early 
period  (see  p.  67),  but  later  when  the  splitting  of  the  embryonic 
mesoderm  takes  place  the  two  cavities  become  continuous  behind 
the  heart,  but  not  anteriorly,  since  the  ventral  wall  of  the  body  is 
formed  in  the  heart  region  before  the  union  can  take  place.  It  is 
possible,  therefore,  to  recognize  two  portions  in  the  embryonic 
coelom,  an  anterior  one,  the  parietal  cavity  (His),  which  is  never 
connected  laterally  with  the  extra-embryonic  cavity,  and  a  posterior 
one,  the  trunk  cavity,  which  is  so  connected.  The  heart  is  situated 
316 


IKE   FERICAaniUM  AND  PLEUXO-PESRONXCII 


317 


in  the  parietal  cavity,  s.  considerable  portion  of  which  is  destined  to 
become  the  pericardial  ca  ^Uy. 

Since  the  pa-ietal    avity  lies  immediately  anterior  to  the  still 
wide  yolk-stalk,  as  -nay  be  seen  from  the  position  of  the  heart  in  the 
embryo  shown  in  Fig.  53,  it  is  bounded 
posterioriy   by   the   yolkstalk.    This 
boundary  is  complete,  however,  only 
in  the  median  line,  the  cavity  being 
continuous  on  either  side  of  the  yolk- 
stalk  with  the  trunk-cavity  by  pas- 
sages which   have  been   termed    the 
recessus  parielales  (Fig.   193,  Bp  and 
Rca).     Passing  forward   toward   the 
heart    in   the   splanchnic   mesoderm 
which  surrounds  the  yolkstalk  are  the 
large  vitelline  veins,  one  on  either  side, 
and  these  shortly  become  so  large  as 
to  bring  the  splanchnic  mesoderm  in 
which  they  lie  in  contact  with  the  so- 
matic mesoderm  which  forms  the  lat- 
eral wall  of  each  recess.    Fusion  of 
the  two  layers  of  mesoderm  along  the 
course  of  the  veins  now  takes  place, 
and  each  recess  thus  becomes  divided 
into  two  parallel  passages,  which  have 
been  termed  the  dorsal  (Fig  194,  rpd) 
and   ventral   (rpv)  parietal  recesses. 
Later  the  two  veins  fuse  in  the  upper 
portion  of  their  course  to  form  the  be- 
ginning of  the  sinus  venosus,  with  the  result  that  the  ventral  re- 
cesses become  closed  below  and  their  continuity  with  the  trunk- 
cavity  is  int.-Tupted,  so  that  they  form  two  blind  pouches  e  ^tending 
downward  a  short  distance  from  the  ventral  portion  of  the  floor  of 
the  parieul  cavity.    The  dorsal  recesses,  howeve     retain  their 
continuity  with  the  trunk-cavity  until  a  much  later  p.jriod. 


Rca 


Fig.  193. — Reconstruction 
OF  A  Rabbit  Eubryo  of  Eight 
Days,  with  the  Pebicahdiai. 
Cavfty  Laid  Open. 

A,  Auricle;  Aob,  aortic  bulb; 
A.V.,  atrio-ventricular communi- 
cation; Bp,  ventral  parietal  re- 
cess;_  Om,  vitelline  vein;  Pc,  peri- 
cardial cavity;  Il£a,  dorsal  pari- 
etal recess;  5v,  sinus  venosus;  V, 
ventricle. — (His.) 


3'8 


THE  PEiicAionm  AMD  pustrxo-nBiToinim 


By  the  fusion  of  the  vitelline  veins  mentioned  above,  there  is 
formed  a  thiclt  semilunar  fold  which  projects  horizontally  into  the 
coelom  from  the  ventral  wall  of  the  body  and  forms  the  floor  of  the 
ventral  part  of  the  parietal  recess.  This  is  known  as  the  septum 
Iransversum,  and  besit'^s  containing  the  anterior  portions  of  the 
vitelline  veins,  it  also  furnishes  a  passage  by  which  the  ductus 
Cuvieri,  formed  by  the  union  of  the  jugular  and  cardinal  veins, 
reach  the  heart.  Its  dorsal  edge  is  continuous  in  the  median  line 
with  the  mesoderm  surrounding  the  digestive  tract  just  opposite 
the  region  where  the  liver  outgrowth  will  form,  but  laterally  this 
edge  is  free  and  forms  the  ventral  iwalls  of  the  dorsal  parietal  recess. 
An  idea  of  the  relations  of  the  septum  at  this  stage  may  be  obtained 


Fro.  194.— Teansvihsi:  Sictions  of  a  RuBtT  Ewnvo  showino  thi  DivaiOH  of 

THB  Parietal  Rxcessis  by  the  Vitiuini  Vsras. 

•in,  Amnioii;  rf,  parietal  receas;  rpd  and  rfv,  dorsal  and  ventral  divisions  of  the  parietal 

recess;  mm,  vitelline  vein. — {Rtttm.) 

from  Fig  195,  which  represents  the  anterior  surface  of  the  septum, 
together  with  the  related  parts,  in  a  rabbit  embryo  of  nine  days. 

The  Separation  of  the  Pericardial  Cavity.— The  septum  trans- 
versum  is  at  first  almost  horizontal,  but  later  it  becomes  decidedly 
oblique  in  position,  a  change  associated  with  the  backward  move- 
merit  of  the  heart.  As  the  closure  of  the  ventral  wall  of  the  body 
extends  posteriorly  the  ventral  edge  of  the  septum  gradually  slips 
downward  upon  it,  while  the  dorsal  edge  is  held  in  its  former  posi- 
tion by  its  attachment  to  the  wall  of  the  digestive  tract  and  the 
ductus  Cuvieri.    The  anterior  surface  of  the  septum  thus  comes  to 


THE   RKICASDICll  AND  FtEUlO-WmiTONlCll  319 

look  ventrally  as  well  as  forward,  and  the  parietal  cavity,  having 
taken  up  into  itself  the  blind  pouches  which  represented  the  ventral 
recesses,  comes  to  lie  to  a  large  extent  ventral  to  the  posterior  recesses. 
As  may  be  seen  from  Fig.  195,  the  ductus  Cuvieri,  as  they  bend 
from  the  lateral  walls  of  the  body  into  the  free  edges  of  the  septum, 
form  a  marked  projection  which  diminishes  considerably  the  open- 
ing of  the  dorsal  recesses  into  the  parietal  cavity.    In  later  stages 


FlO.  I9S.-1UC0N8T«DCTI0N  FROM  A  RabBIT  EhBEYO  OP  NiNI  DAYS  SHOWINO  Tm 

Septum  TuArJsviEsuH  peoh  Abovi. 
am,  Amnion;  at,  atrium;  dc,  ductus  Cuvieri;  rpd,  doraal  parietal  recess.— (Satn.) 

this  projection  increases  and  from  its  dorsal  edge  a  fold,  which 
may  be  regarded  as  a  continuation  of  the  free  edge  of  the  septum, 
projects  into  the  upper  portions  of  the  recesses  and  eventually  fuses 
with  the  median  portion  of  the  septum  attached  to  the  wall  of  the  gut. 
In  this  way  the  parietal  cavity  becomes  a  completely  closed  sac,  and 
is  henceforward  known  as  the  pericardial  cavity,  the  original  ccelom 


fill 


I'i 


3*0 


THS   DIAPHIAOM 


being  now  divided  into  two  portions,  (i)  the  pericardial,  and  (a)  the 
pkuroperiKmeal  cavities,  the  latter  consisting  of  the  abdominal 
coelom  together  with  the  two  dorsal  parietal  recesses  which  have 
been  separated  from  the  pericardial  (parietal)  cavity  and  are  des- 
tined to  be  converted  into  the  pleural  cavities. 

The  FormatUm  of  the  Diaphragm.— \\.  is  to  be  remembered  that 
the  attachment  of  the  transverse  septum  to  the  ventral  wall  of  the 
digestive  tract  is  opposite  the  point  where  the  liver  outgrowth 
develops.    When,  therefore,  the  outgrowth  appears,  it  pushes  its 


^1 


ir.„   T^     ntAORAiis  or  (/I)  A  &*oiTTAi  Section  or  an  Embryo  showing  tb» 
U,  umbilicus. 

way  into  the  substance  of  the  septum,  which  thus  acquires  a  very 
considerable  thickness,  especially  toward  its  dorsal  edge,  and  it 
furthermore  becomes  diJEerentiated  into  two  layers,  an  upper  one, 
which  forms  the  floor  of  the  ventral  portion  of  the  pericardial  cavity 
and  encloses  the  Cuvierian  ducts,  and  a  lower  one  which  contains  the 
liver.  The  upper  layer  is  comparatively  thin,  while  the  lower  forms 
the  greater  part  of  the  thickness  of  the  septum,  its  posterior  surface 
meeting  the  ventral  wall  of  the  abdomen  at  the  level  of  the  antenor 
margin  of  the  umbilicus  (Fig.  196,  A). 


XBK   OIAVHSAOM 


3»i 


In  later  stages  of  development  the  layer  containing  the  liver 
becomes  separated  fiom  the  upper  layer  by  two  grooves  which, 
appearing  at  the  sides  and  ventrally  immediately  over  the  liver 
(Fig.  196,  B),  gradually  deepen  toward  the  median  line  and  dorsally. 
These  grooves  do  not,  however,  quite  reach  the  median  line,  a  por- 
tion of  the  lower  layer  of  the  septum  being  left  in  this  region  as  a 
fold,  situated  in  the  sagittal  plane  of  the  body  and  attached  above 
to  the  posterior  surface  of  the  upper  layer  and  below  to  the  anterior 
surface  of  the  liver,  beyond  which  it  is  continued  down  the  ventral 
wall  of  the  abdomen  to  the  umbilicus  (Fig.  ig6,  C,Ls).  This  is  the 
jakijorm  ligament  of  the  liver  of  adult  anatomy,  and  in  the  free 
edge  of  its  prolongation  down  the  ventral  wall  of  the  abdomen  the 
umbilical  vein  passes  to  the  under  surface  of  the  liver,  while  the  free 
edge  of  that  portion  which  lies  between  the  liver  and  the  digestive 
tract  contains  the  vitelline  (portal)  vein,  the  common  bile-duct,  and 
the  hepatic  artery.  The  diagram  given  in  Fig.  196  will,  it  is  hoped, 
make  clear  the  mode  of  formation  and  the  relation  of  this  fold, 
which,  in  its  entirety,  constitutes  what  is  sometimes  termed  the 
ventral  mesentery. 

And  not  only  do  the  grooves  fail  to  unite  in  the  median  line,  but 
they  also  fail  to  completely  separate  the  liver  from  the  upper  layer 
of  the  septum  dorsally,  the  portion  of  the  lower  layer  which  persists 
in  this  region  forming  the  coronary  ligament  of  the  liver.  The 
portion  of  the  lower  layer  which  forms  the  roof  of  the  grooves  be- 
comes the  layer  of  peritoneum  covering  the  posterior  surface  of  the 
upper  layer  (which  represents  the  diaphragm),  while  the  portion 
which  remains  connected  with  the  liver  constitutes  its  peritoneal 
investment. 

In  the  meantime  changes  have  been  taking  place  in  the  upper 
layer  of  the  septum.  As  the  rotation  of  the  heart  occurs,  so  that  its 
atrial  portion  comes  to  lie  anterior  to  the  ventricle,  the  Cuvierian 
ducts  are  drawn  away  from  the  septum  and  penetrate  the  posterior 
uull  of  the  pericardium,  the  separation  being  assisted  by  the  con- 
linued  descent  of  the  attacbnent  of  the  edge  of  the  septum  to  the 
vcntra'  wall  of  the  bodv  ing  the  descent,  when  the  upper 


fit 


3'» 


THE   PIEtnt^ 


layer  of  the  septum  has  reathed  the  level  of  the  fourth  cervical  seg- 
lli   H  ment,  portions  of  the  myotomes  of  that  segment  become  prolonged 

into  it  and  the  layer  assumes  the  characteristics  of  the  diaphragm, 
the  supply  of  whose  musculature  from  the  fourth  cervical  nerves  is 
thus  explained. 

The  Pleura.— The  diaphragm  is  as  yet,  however,  incomplete 
dorsally,  where  the  dorsal  parieUl  recesses  are  still  in  continuity  with 
the  trunk-cavity.  With  the  increase  in  thickness  of  the  septum 
transversum,  these  recesses  have  acquired  a  considerable  length 
antero-posteriorly,  and  into  their  upper  portions  the  outgrowths 
from  the  lower  part  of  the  pharynx  which  form  the  lungs  (see  page 
331)  begin  to  project.  Tl*  recesses  thus  become  transformed 
into  the  pleural  cavities,  and  as  the  diaphragm  continues  to  descend, 
slipping  down  the  ventral  wall  of  the  body  and  drawing  with  it  the 
pericardial  cavity,  the  latter  comes  to  lie  entirely  ventral  to  the  pleural 
cavities.  The  free  borders  of  the  diaphragm,  which  now  form  the 
ventral  boundaries  of  the  openings  by  which  the  pleural  and  peri- 
toneal cavities  communicate,  begin  to  approach  the  dorsal  wall  of 
the  body,  with  which  they  finally  unite  and  so  complete  the  separa- 
tion of  the  cavities.  The  pleural  cavities  continue  to  enlarge  after 
their  separation  and,  extending  laterally,  pass  between  the  peri- 
cardium and  the  lateral  walls  of  the  body  until  they  finally  almost 
completely  surround  the  pericardium.  The  intervals  between  the 
two  pleurae  form  what  are  termed  the  mediastina. 

The  downward  movement  of  the  septum  transversum  extends 
through  a  very  considerable  interval,  which  may  be  appreciated 
from  the  diagram  shown  in  Fig.  197-  From  this  it  may  be  seen 
that  in  early  embryos  the  septum  is  situated  just  in  front  of  the  first 
cervical  segment  and  that  U  lies  very  obliquely,  its  free  edge  being 
decidedly  posterior  to  its  ventral  attachment.  When  the  downward 
displacement  occurs,  the  ventral  edge  at  first  moves  more  rapidly 
than  the  dorsal,  and  soon  comes  to  lie  at  a  much  lower  level.  The 
backward  movement  continues  throughout  the  entire  length  of  the 
cervical  and  thoracic  regions,  and  when  the  level  of  the  tenth  tho 
racic  segment  is  reached  the  separation  of  the  pleural  and  peritoneal 


THE    PESITONEUIC 


3'3 


cavities  IS  completed,  and  then  the  dorsal  edge  begins  to  descend 
more  rapidly  than  the  ventral,  so  that  the  diaphragm  again  becomes 
obhque  m  the  same  sense  as  in  the  beginning,  a  position  which  it 
retains  m  the  adult. 

The  Development  of  the  Pertioneum.-The  peritoneal  cavity  is 

developed  from  the  trunk-cavity  of  early  stages  and  is  at  first  in  free 

communication  on  all   sides  of  the- 

yolk-stalk  with  the   extra-embryonic 

coelom.    As  the  ventral  wall  of  the 

body  develops  the  two  cavities  become 

more  and  more  separated,  and  with 

the  formation  of  the  umbilical  cord 

the   separation   is   complete.    Along 

the  middorsal  line  of  the  body  the 

archenteron  forms  a  projection  into 

the  cavity  and  later  moves  further  out 

from  the  body-wall  into  the  cavity, 

pushing  in  front  of  it  the  peritoneum, 

which  thus  comes  to  surround  the  in- 
testine, forming  iu  serous  coat,  and 
from  it  is  continued  back  to  the  dorsal 
body-wall  forming  the  mesentery. 

It  has  already  been  seen  that  on 
the  separatioa  of  the  liver  from  the 
septum  transversum,  the  tissue  of  the 
latter  gives  rise  to  the  peritoneal 
covering  of  the  liver  and  of  the  pos- 
terior surface  of  the  diaphragm,  and  also  to  the  ventral  mesentery 
When  the  separation  is  taking  place,  the  rotation  of  the  stomach  al- 
ready described  (p.  301)  occurs,  with  the  result  that  the  portion  of  the 
ventral  mesentery  which  stretches  between  the  lesser  curvature  of  the 
stomach  and  the  liver  shares  in  the  rotation  and  comes  to  lie  in  a  plane 
practically  at  right  angles  with  that  of  the  suspensory  ligament,  its  sur- 
faces looking  dorsally  and  ventrally  and  its  free  edge  being  directed 
toward  the  right    This  portion  of  the  ventral  mesentery  forms 


FlO.    197.— DtAGKAlI  SBOWINO 
THI  POSmON  OF  IHE  DlAPaKACH 

IS  Ehbryos  of  Diffejimi  Aoes 
—(Matt.) 


.r' 


3«4 


THE   FEKHONEtril 


what  is  termed  the  \user  omtntum,  and  between  it  and  the  dorsrf 
surface  of  the  stomach  as  the  ventral  boundaries,  and  the  dorsal 
wall  of  the  abdominal  cavity  dorsally,  there  is  a  cavity,  whose  floor 
is  formed  by  the  dorsal  mesentery  of  the  stomach,  the  mesogastrtum, 
the  roof  by  the  under  surface  of  the  left  half  of  the  liver,  while  to  the 
right  it  communicates  with  the  general  peritoneal  cavity  dorsal  to 
the  free  edge  of  the  lesser  omentum.  This  cavity  is  known  as  the 
bursa  ommuau  Cesser  sac  of  the  peritoneum),  and  the  opemng  into 
it  from  the  general  cavity  or  greater  sac  is  termed  the  epitiotcforamm 
(foramen  of  Winslow).  Later,  the  floor  of  the  lesser  sac  is  drawn 
downward  to  form  a  broad  sheet  of  peritoneum  lying  ventral  to  the 
coUs  of  the  small  intestine  and  consisting  of  four  layers;  this  repre- 
sents the  great  omentum  of  adult  anatomy  (Fig.  201). 

Although  the  form  assumed  by  the  bursa  omentalis  is  assoaated 
with  the  rotation  of  the  stomach,  it  seems  probable  that  its  real 
origin  is  independent  of  that  process  (Broman).    The  subserous 
tissue  of  the  transverse  septum  is  at  first  thick  and  includes  not  only 
the  liver,  but  alsothe  pancreas  and  the  portion  of  the  dig«tive  tract 
which  becomes  the  stomach  and  the  upper  part  of  the  duodenum 
(Fie  106  A)     The  shrinkage  of  this  tissue  by  which  these  organs 
become  ^parated  from  the  septum  cannot  take  place  evenly  on 
account  of  the  relations  which  the  organs  bear  to  one  another,  so 
that  on  the  right  side  certain  peritoneal  recesses  are  formed,  one 
between  the  right  lung  and  the  stomach,  a  second  between  the  liver 
and  the  stomach,  and  a  third  between  the  pancreas  and  the  same 
structure     In  man  these  three  recesses  communicate  with  one 
another  to  form  the  primary  bursa  omentalis,  and  open  by  a  com- 
mon epiploic  foramen  into  the  general  peritoneal  cavity.    The  rota- 
tion of  the  stomach,  which  takes  place  later,  merely  serves  to  modify 
the  original  bursa. 

Tn  A,f  human  embrvo  a  small  recess  also  forms  upon  the  left  side 
bet^^n  tiie  kTfu^rand  the  stomach.  Later  it  separates  from  the  rest 
of  afw  oment^s  and  passes  up  along  the  side  of  the  oKophagus 
^^g  to^e  on  its  right  side  between  it  and  the  diaphragm^  I  pve.  ns 
toTImaU  «rou.  sac  that  Ues  beneath  the  infracardial  lobe  of  the  right 


THE   VEKItatrEUU 


32s 


lung,  when  this  is  present,  and  hence  has  been  termed  the  infracardial 
bursa. 

Below  the  level  of  the  upper  part  of  the  duodenum  the  ventral 
mensentery  is  wanting;  only  the  dorsal  mesentery  occurs.  So  long 
as  the  intestine  is  a  straight  tube  the  length  of  the  intestinal  edge  of 
this  mesentery  is  practically  equal  to  that  of  its  dorsal  attached  edge. 
The  intestine,  however,  increasing  in  length  much  more  rapidly 
than  the  abdominal  walls,  the  intestinal  edge  of  the  mesentery  soon 
becomes  very  much  longer  than  the  at- 
tached edge,  and  when  the  intestine  grows 
out  into  the  umbilical  ccelom  the  mesentery 
accompanies  it  (Fig.  198).  As  the  coils  of 
the  intestine  develop,  the  intestinal  edge  of 
the  mesentery  is  throwji  into  corresponding 
folds,  and  on  the  return  of  the  intestine  to 
the  abdominal  cavity  the  mesentery  is 
thrown  into  a  somewhat  funnel-like  form 
by  the  twisting  of  the  intestine  to  form  its 
primary  loop  (Fig.  199).  All  that  portion 
of  the  mesentery  which  is  attached  to  the 
part  of  the  intestine  which  will  later  become 
the  jejunum,  ileum,  ascending  and  trans- 
verse colon,  is  attached  to  the  body-wall 
at  the  apex  of  the  funnel,  at  a  point  which  b*™  of  Sa  Weeks 
lies  to  the  left  of  the  duodenum.  . /■  P""™^-;  ^,  f"^^- 

Sp,  spleen. — i^Totdi.) 

Up  to  this  Stage  or  to  about  the  middle 
of  the  fourth  month  the  mesentery  has  retained  its  attachment  to  the 
median  line  of  the  dorsal  wall  of  the  abdomen  throughout  its  entire 
length,  but  later  fusions  of  certain  portions  occur,  whereby  the  orig- 
inal condition  is  gret  Jy  modified.  One  of  the  earliest  of  these  fusions 
takes  place  at  the  apex  of  the  funnel,  where  the  portion  of  the  mesen- 
tery which  passes  to  the  tranverse  colon  and  arches  over  the  duo- 
denum fuses  with  the  ventral  surface  of  the  latter  portion  of  the 
intestine  and  also  with  the  peritoneum  covering  the  dorsal  wall  of  the 
abdomen  both  to  the  right  and  to  the  left  of  the  duodenum.    In  this 


Fig.    198. — DiAGKAU 

SHOWING  THE  AbKANGEUENT 

OF  THE  Mesentery  and  Vis- 
ceral Branches  of  the  Ab- 
dominal Aorta  in  ah  Eh- 


336  THE   FEKITONEtni 

way  the  attachment  of  the  transverse  mesocolon  takes  the  form  of  a 
transverse  line  instead  of  a  point,  and  this  portion  of  the  mesentery 
divides  the  abdominal  cavity  into  two  portions,  the  upper  (anterior) 
of  which  contains  the  liver  and  stomach,  while  the  lower  contains 
the  remainder  of  the  digestive  tract  with  the  exception  of  the  duo- 
denum. By  passing  across  the  ventral  surface  of  the  duodenum 
and  fusing  with  it,  the  transverse  mesocolon  forces  that  portion  of 
the  intestine  against  the  dorsal  wall  of  the  abdomen  and  fixes  it  in 
that  position,  and  its  mesentery  thereupon  degenerates,  becoming 


auf 


FlO.    199. — DiAOKAlIS  IlLDSIIATINO  IHE  DEVIIOPMKNT  OI  THE   GxIA.    OUNTUU 
AND  THI  TiANSVISSI  MESOCOLON. 

bid,  QEcum;  dd,  small  intiatiiK;  dg,  yolk-stalk;  di,  coton;  du,  duodenum;  fe,  greater 
curvature  of  stomach;  «,  bile  duct;  pi,  mesogastrium;  k,  point  where  the  loops  of  Uie 
intestine  cross;  mc,  moocolon;  md,  rectum;  dm,  mesenteiy;  wf,  vermiform  appendix. 
— (Ha-ftOTf.) 

subserous  areolar  tissue,  the  duodenum  assuming  the  retroperito- 
neal position  which  characterizes  it  in  the  adult. 

The  descending  colon,  which  on  account  of  the  width  of  its  mes- 
entery is  at  first  freely  movable,  lies  well  over  to  the  left  side  of  the 
abdominal  cavity,  and  in  consequence  the  left  layer  of  its  mesentery 
lies  in  contact  with  the  parietal  layer  of  the  peritoneum.  A  fusion 
of  these  two  layers,  beginning  near  the  middle  line  and  thence  extend- 
ing outward,  takes  place,  the  fused  layers  becoming  converted  into 


THE   FEKnONEUM 


38? 


connective  tissue,  and  this  portion  of  the  colon  thus  loses  its  mesen- 
tery and  becomes  fixed  to  the  abdominal  wall.  The  process  by 
which  the  fixation  is  accomplished  may  be  understood  from  the 
diagrams  which  constitute  Fig.  ioo.  When  the  ascending  colon  is 
formed,  its  mesentery  undergoes  a  similar  fusion,  and  it  also  becomes 
fixed  to  the  abdominal  wall. 

The  fusion  of  the  mesentery  of  the  ascending  and  descending  colon 
teniains  incomplete  in  a  considerable  number  of  cases  (one-fourth  to  one- 
third  of  all  cases  examined),  and  in  these  the  colons  are  not  perfectly 
fixed  to  the  abdominal  wall.  It  may  also  be  pointed  out  that  the  cKcum 
and  appendix,  being  primarily  a  lateral  outpouching  of  the  intestine,  do 


Fro.  200.-DU0BAKS  iLLnSTHATINO  TBI  MANNER  DJ  WHICH  THE  FkaIION  OF  THE 

Descendino  Colon  (C)  takes  Place. 

not  possess  any  true  mesentery,  but  are  completely  enclosed  by  peritoneum. 
UsuaUy  a  f  Jciform  fold  of  peritoneum  may  be  found  extending  along  one 
surface  of  the  appendix  to  become  continuous  with  the  left  layer  of  the 
mesentery  of  the  ileum.  This,  however,  is  not  a  true  mesentery,  and  is 
better  spoken  of  as  a  mesenteriole. 

One  other  fusion  is  still  necessary  before  the  adult  condition  of 
the  mesentery  is  acquired.  The  s.  ;.  i  omentum  consists  of  two 
folds  of  peritoneum  which  start  froLa  the  greater  curvature  of  the 
stomach  and  pass  downward  to  be  reflected  up  again  to  the  dorsal 
wall  of  the  abdomen,  which  they  reach  just  anterior  to  (above)  the 
line  of  attachment  of  the  transverse  mesocolon  (Fig.  201,  A).    At 


3»8 


THE  nutrroNEuii 


I 


first  the  attachment  of  the  omentum  is  vertical,  since  it  represents 
the  mesogastrium,  but  later,  by  fusion  with  the  parietal  peritoneum, 
it  assumes  a  transverse  direction,  while  at  the  same  time  the  pancreas, 
which  originally  lay  between  the  two  folds  of  the  mesogastrium,  is 
carried  dorsally  and  comes  to  have  a  retroperitoneal  position  in  the 
line  of  attachment  of  the  omentum.  By  this  change  the  lower  layer 
of  the  omentum  is  brought  in  contact  with  the  upper  layer  of  the 


A  I  B 

FlO.  aoi. — DlAGBAHS  SHOWIHO  THE  DeVXLOPHEHT  Or  TEX  GUAT  OlfXNTUV  AND  rrs 

Fdsion  with  the  TiANSvnsi  Mxsocolok. 

B,  Bladder;  c,  transverse  colon;  d,  duodenum;  Li^  liver;  f,  pamzieas;  R,  rectum;  5, 

stomach;  U,  uterus. — (.After  Attpi  Thamsm.) 

transverse  mesocolon  and  a  fusion  and  degeneration  of  the  two  re- 
sults (Fig.  20I  B),  a  condition  which  brings  it  about  that  the  omen- 
tum seems  to  be  attached  to  the  transverse  colon  and  that  the  pan- 
creas seems  to  lie  in  the  line  of  attachment  of  the  transverse  meso- 
colon. This  mesentery,  as  is  occurs  in  the  adult,  really  consists 
partly  of  a  portion  of  the  original  transverse  mesocolon  and  partly 
of  a  layer  of  the  great  omentum. 


UTEKATintZ 


3*9 


By  these  various  changes  the  line  of  attachment  of  the  mesen- 
tery to  the  dorsal  wall  of  the  body  has  become  somewhat  compli- 
cated and  has  departed  to  a  very  considerable  extent  from  its  origi- 
nal simple  vertical  arrangement.    If  all  the  viscera  be  removed 
from  the  body  of  an  adult  and  the  mesentery  be  cut  close  to  the  line 
of  its  attachment,  the  course  of  the  latter  will  be  seen  to  be  as  fol- 
lows:   Descending  from  the  under  surface  of  the  diaphragm  are 
the  lines  of  attachment  of  the  suspensory  ligament,  which  on 
reaching  the  liver  spread  out  to  become  the  coronary  and  lateral 
ligaments  of  that  organ.    At  about  the  mid-dorsal  line  these  lines 
become  continuous  with  those  of  the  mesogastrium  which  curve 
downward  toward  the  left  and  are  continued  into  the  transverse  Unes 
of  the  transverse  mesocolon.    Between  these  last,  in  a  slight  prolonga- 
tion, there  may  be  seen  to  the  right  the  cut  end  of  the  first  portion 
of  the  duodenum  as  it  passes  back  to  the  dorsal  wall  of  the  abdomen, 
and  at  about  the  mid-dorsal  line  the  cut  ends  of  its  last  part  become 
visible  as  it  passes  ventrally  again  to  become  the  jejunum.    From  the 
transverse  mesocolon  three  lines  of  attachment  pass  downward;  the 
two  lateral  broad  ones  represent  the  lines  of  fixation  of  the  ascending 
and  descending  colons,  while  the  narrower  median  one,  which 
curves  to  the  right,  represents  the  attachment  of  the  mesentery  of 
the  small  intestine  other  than  the  duodenum.    Finally,  from  the 
lower  end  of  the  fixation  line  of  the  descending  colon  the  mesentery 
of  the  sigmoid  is  continued  downward. 

The  special  developments  of  the  peritoneum  in  connection  with 
the  genito-urinary  apparatuus  will  be  considered  in  Chapter  XIII. 

LITERATURE. 

I.   Bioman:  "Ueber  die  EBtwicklung  und  Bedcutung  der  Mesenterial  und  d« 

KSrperhaMen  bei  den  Wirbeltieren,"  Ergdm.  der  Anal.  u.  Entw.,  xv,  1906. 
A.  Beachct:  "Die  Entwickelung  der  grossen  K8rperh6hlen  und  ihre  T-ennung  von 

Emander,"  ErgOnUa  der  Altai,  imd  Bthuickelungsgesck.,  Vll,  1898. 
W.  His:  "MittlieUungen  zur  Embiyologie  der  Siugethiere  imd  des  Menjchen  " 

Archhfar  Anat.  md  Pkysiol.,  Anal.  Ahth.,  1881. 
F.  P.  Maix:  "Development  of  the  Human  Coelom,"  Journal  0/ Morfhal.,  Ml,  1897. 
F.  P.  Mali:  "On  the  Development  ol  the  Human  Diaphragm,"  Johns  Hopkins 

Hospital  Bull.,  joi,  1901. 


33° 


LmiATtnuc 


% 


E.  Ravn;  "Uefaer  dia  BUdung  der  Scheideinuid  nrbcbB  Bnut-  and  BMchhShk  b 

StugediiernnbiToiMn,"  Arckhi  fm  Aaat.  tmi  PkytU.,  Anal,  AM.,  1889. 
A.  Swain:  "Rcchetchn  lur  le  dMoppemenl  du  foie,  du  tube  digatU,  de  l"«na«- 

cmvlK  du  peritoiae  et  du  mtenlto,"  Jamn.  dt  TAwU.  it  lit  la  Pkytlcl.,  xxxn, 

1I96;  xxxin,  1897. 
C.  Toujt:  "B»u  und  Wachstttnuveriinderuiigen  der  GekiflK  da  mouchlkhen 

Dumkuuk,"  Dmbdr.  dm  tats.  Akad.  WUimuh.  Wim,  M(M.-Nalmwist. 

Clatu,  XLI,  1879.  ' 

C.  ToLBi:  "Die  Dumgduflw  und  Net»  Im  geKbnusuien  und  gcietiwidijga 

Zustand,"  Dmktckr.  der  ktis.  Akad.  Wismuck.  Witn.  Ualk.-Nalmwm.  Ckuu, 

LVI,  1889. 

F.  Tuvn:  "Lectutet  on  the  Anatomjr  of  the  Inteitinnl  C*a*l  ud  Perilgoenn," 

Brilisk  Utdical  Jomal,  I,  1885. 


CHAPTER  XII. 

THB  DEVELOPMEHT  OF  THB  ORGAIIS  OF  RESPIRATIOH. 

The  DeTelopment  of  th«  Luii«..-The  first  indication  of  the 
lungs  and  trachea  is  found  in  embryos  of  about  3.2  mm.  in  the 
form  of  a  groove  on  the  ventral  surface  of  the  oesophagus,  at  first  ei- 
tendmg  almost  the  entire  length  of  that  portion  of  the  digestive 
tract  ^  the  oesophagus  lengthens  the  lung  groove  remains  con- 
nected with  Its  upper  portion  (Fig.  182,  A),  and  furrows  which  ap- 
pear along  the  line  of  junction  of  the  groove  and  the  oesophagus 
gradually  deepen  and  separate 
the  two  structures  (Fig.  i8a,B). 
The  separation  takes  place  earliest 
at  the  lower  end  of  the  groove 
and  thence  extends  upward,  so 
that  the  groove  is  transformed 
into  a  cylindrical  pouch  lymg  ven- 
tral to  the  oesophagus  and  dorsal 
to  the  heart  and  opening  with  the 
•oesophagus  into  the  terminal  por- 
tion of  the  pharynx. 

Soon  after  the  separation  of 
the  groove  from  the  oesophagus 
its  lower  end  becomes  enlarged 
and  bllobed,  and  since  this  lower 
end  lies,  with  the  oesophagus,  in 
the  median  attached  portion  of  the  dorsal  edge  of  the  septum  trans- 
versum,  the  lobes,  as  they  enlarge,  project  into  the  dorsal  parietal 
recesses  (Fig.  202),  and  so  become  enclosed  within  the  peritoneal 
lining  of  the  recesses  which  later  become  the  pleural  cavities. 
The  lobes,  which  represent  the  lungs,  do  not  long  remain  simple, 
33' 


MI.— POUTION    or  A  SicrioN 

THKOUGH  AN  EhbRYO  OF  THE  FOURTH 

Week. 

A,  Aorta;  DC,  ductus  Cuvieri;  L, 
lung;  O,  cEsophagus;  RP,  parietal  re- 
cess; VOm,  vitelline  vein.— (rofcft.) 


33* 


IBB  Limos 


but  bud-like  processes  arise  from  their  cavities,  three  appearing  in 
the  right  lobe  and  two  in  the  left  (Fig.  203,  A),  and  as  these  increase 
in  size  and  give  rise  to  additional  outgrowths,  the  structtire  of  the 
lobes  rapidly  becomes  complicated  (Fig.  203,  B  and  C). 

The  lower  primary  process  on  each  side  may  be  regarded  as  a 
prolongation  of  the  bronchus,  while  the  remaining  process  or  pro- 
cesses represent  lateral  outgrowths  from  it.  Considerable  difference 
of  opinion  has  existed  as  to  the  nature  of  the  further  branching  of  the 
bronchi,  some  authors  regarding  it  as  a  succession  of  dichotomies, 
one  branch  of  each  of  these  placing  itself  so  as  to  be  in  the  line  of  the 


«  C 

Fro.  903. — RXCONSTXUCTXON  Of  THE  LUHO  Ol^TOROWTHS  OF  EMBRYOS  OF  {A)  4,3, 

(B)  8.S,  AMD  (Q  10.5  lOL 

Ap,  Fulmonaiy  artoy;  Et,  epaiterul  bionchiu;  Vf,  pulmonary  vdn;  I,  second  lateral 

bronchus;  //,  main  bronchi. — {His,) 

original  main  bronchus,  while  the  other  comes  to  resemble  a  lateral 
outgrowth,  and  other  observers  have  held  that  the  main  bronchus 
has  an  uninterrupted  _  owth,  all  other  branches  being  lateral  out- 
growths from  it,  and  the  branching  therefore  a  monopodial  process. 
The  recent  thorough  study  by  Flint  of  the  development  of  the  lung  of 
the  pig  shows  that,  in  that  form  at  least,  the  branching  is  a  mono- 
podial one,  and  that  from  the  main  bronchus  as  it  elongates  four  sets 
of  secondary  outgrowths  develop,  namdy,  a  strong  lateral,  a  dorsal, 
a  ventral,  and  a  weak  and  variable  medial  set 


I.UNOI 


333 


There  is  a  general  tendency  for  the  individual  branches  of  the 
various  sets  to  be  arranged  in  regular  succession  and  for  their  develop- 
ment to  be  symmetrical  in  the  two  lungs.  But  on  account  of  the 
necessity  under  which  the  lungs  are  placed  of  adapting  themselves 
to  the  neighboring  structures  and  at  the  same  time  afifording  a 
respiratory  surface  as  large  as  possible,  an  amount  of  asymmetry 
supervenes.  Thus,  it  has  already  been  noted  that  in  the  earliest 
branching  a  single  lateral  bronchus  is  formed  in  the  left  lung  and  two 
in  the  right.  The  uppermost  of  these 
latter,  the  first  lateral  bronchus,  is  un- 
represented in  the  left  lung,  and  is  pecu- 
liar in  that  it  lies  behind  the  right  pul- 
monary artery  (Fig.  203,  C),  or  in  the 
adult,  after  the  recession  of  the  heart, 
above  it,  whence  it  is  termed  the  epar- 
tarial  bronchus.  Its  absence  on  the  left 
side  is  perhaps  due  to  its  suppression  to 
permit  the  normal  recession  of  the  aortic 
arch  (Flint). 

So,  too,  the  inclination  of  the  heart 
causes  a  suppression  of  the  second  ven- 
tral bronchus  in  the  left  lung,  but  at 
the  same  time  it  affords  opportunity  for 
an  excessive  development  of  the  corre- 
sponding bronchus  of  the  right  lung, 
which  pushes  its  way  between  the  heart 
and  the  diaphragm  and  is  known  as  the 
infra-cardiac  bronchus. 

As  soon  as  the  unpaired  first  lateral  bronchus  and  the  paired  sec- 
ond lateral  bronchi  are  formed  mesenchyme  begins  to  collect  around 
each  of  them  and  also  around  the  main  bronchi,  the  lobes  of  the 
adult  lung,  three  in  the  right  lung  and  two  in  the  left,  being  thus 
outlined.  A  development  of  mesenchyme  also  takes  place  around 
the  excessively  developed  right  second  ventral  bronchus,  and  some- 
times produces  a  well-marked  infra-cardiac  lobe  in  the  right  lung. 


FiC.  204. — DlAGKAlf  OF  TH« 

Final  Bkanchzs  of  thk  Mau- 

HALIAN  BKONCHI. 

A,  Atrium;  B,  bronchus;  S, 
*ir-sac.— (Afillir.) 


334 


1RX  LAIYNX 


In  Uter  stagf  ;he  various  bronchi  of  each  lobe  give  rise  to 
additional  branches  and  these  again  to  others,  and  the  mesenchyme 
of  each  lobe  grows  in  between  the  various  branches.  At  first  the 
amount  of  mesenchyme  separating  the  branches  is  comparatively 
great,  but  as  the  branches  continue,  the  growth  of  the  mesenchyme 
fails  to  keep  pace  with  it,  so  tlut  in  later  stages  the  terminal  enlarge- 
ments are  separated  from  one  another  by  only  very  thin  partitions 
of  mesenchyme,  in  which  the  pulmonary  vessels  form  a  dense  net- 
work. The  final  branching  of  each  ultimate  bronchus  or  bronchiole 
results  in  the  formation  at  its  extremity  of  from  three  to  five  enlarge- 
ments, the  alria  (Fig.  204,  A),  from  which  arise  a  number  of  air-sacs 
(S)  whose  walls  are  pouched  out  into  slight  diverticula,  the  air-cells 
or  alvadi.    Such  a  combination  of  atria,  air-sacs,  and  air-cells 

'Constitutes  a  lobule,  and  each  lung 
is  composed  of  a  large  number  of 
such  units. 

The  greater  part  of  the  origi- 
nal  pulmonary  groove   becomes 
converted  into  the  trachea,  and  in 
the  mesenchyme  surrounding  it 
the  incomplete  cartilaginous  rings 
develop  at  about  the  eighth  or 
ninth  week.    The  cells  of  the  epi- 
thelial lining  of  the  trachea  and 
bronchi  remain  columnar  or  cu- 
bical in  form  and  become  ciliated 
at  about  the  fourth  month,  but 
those  of  the  epithelium  of  the  air- 
sacs  become  greatly  flattened  and 
constitute    an    exceedingly    thin 
layer  of  pavement  epithelium. 
The  Development  of  the  Larjmz.— The  opening  of  the  upper 
end  of  the  pulmonary  groove  into  the  pharynx  is  situated  at  first 
just  behind  the  fourth  branchial  furrow  and  is  surrounded  anterioriy 
and  laterally  by  the  D-shaped  ridge  already  described  (p.  294)  as 


FlO.  a05. — RECONBTSUCnON  of  th« 
OpININO  WTO  THI  LaiYNX  DI  AM  EK- 
BMYO  OF  TWINTY-MOHT  DAYS,  SuH 
FROM  BiHIHO  AMD  AbOVX,  THI  DOUAL 

Waix  of  ih«    Fhakymx  Bimo  Cdi 
Away. 

CO,  Corniculu',  and  eu,  cuneiform  tu- 
bercle; Bf,  epiglottu;  T,  unpaired  por- 
tion of  the  tongue.— (JCoUilu.) 


nn  LAxyifx 


335 


the  furcula,  this  tepanting  it  from  the  posterior  portion  of  the 
tongue  (Fig.  178).  The  anterior  portion  of  this  ridge,  which  is 
apparently  derived  from  the  ventral  portions  of  the  third  branchial 
arch,  gradually  increases  in  height  and  forms  the  epigUtUis,  while 
the  lateral  portions,  which  pass  posteriorly  into  the  margins  of  the 
pulmonary  groove,  form  the  aryepighUU  folds.  When  the  pulmon- 
ary groove  separates  from  the  oesophagus,  the  opening  of  the  trachea 
into  the  pharynx  is  somewhat  slit-like  and  is  bounded  laterally  by 
the  aryepiglottic  folds,  whose  margins  present  two  elevations  which 
may  be  termed  the  comicular  and  cuneiform  tubtrcUs  (Fig.  205,  co 
and  cu,  and  Fig.  175).  The  opening  is,  however,  for  a  time  almost 
obliterated  by  a  thickening  of  the  epithelium  covering  the  ridges, 


'"••  »06.— RlCO>l»T»DCno«  01  TBI  MUENCHYU  CoHDXHIATIONS  WHICB  RlPUSINT 
THl  HYOIS  AKD  TBYUOm  CAinUOtl  IN  AN  EliBltYO  OF  FoiTY  D*Y». 

The  darkly  shaded  ueH  repreient  centen  of  cbondrlfiution.    c.ma,  Greater  comu  of 
hydd;  e.mi,  tester  comu;  Th,  thyreoid  cartilage.— (JCoaiiu.) 

and  it  is  not  until  the  tenth  or  eleventh  week  of  development  that 
it  is  re-established.  Later  than  this,  at  the  middle  of  the  fourth 
month,  a  linear  depression  makes  its  appearance  on  the  mesial 
surface  of  each  aryepiglottic  fold,  forming  the  beginning  of  the 
ventricle,  and  although  at  first  the  depression  lies  horizontally,  its 
lateral  edge  later  bends  anteriorly,  so  that  its  surfaces  look  outward 
and  inward.  The  lips  which  bound  the  opening  of  the  ventricle 
into  the  laryngeal  cavity  give  rise  to  the  ventricular  and  vocal  folds. 
The  cartilages  of  the  larynx  can  be  distinj^hed  during  the 
seventh  week  as  condensations  of  mesenchyme  which  are  but 
indistinctly  separated  from  one  another.  The  thyreoid  cartilage  is 
represented  at  this  stage  by  two  lateral  plates  of  mesenchyme, 


336 


IHE   LAKYNX 


separated  from  one  another  both  ventrally  and  dorsally,  and  each 
of  these  plates  undergoes  chondrification  from  two  separate  centers 
(Fig.  206).  These,  as  they  inorease  in  size,  unite  together  and  send 
prolongations  ventrally  which  meet  in  the  nid-ventral  line  with  the 
corresponding  prolongations  of  the  plates  of  the  opposite  side,  so 
as  to  enclose  an  area  of  mesenchyme  into  which  the  chondrification 
only  extends  at  a  later  period,  and  occasionally  fails  to  so  extend, 
producing  what  is  termed  a  foramen  thyreoideum. 

The  mesenchymal  condensations  which  represent  the  cricoid 
and  arytenoid  cartilages  are  continuous,  but  each  arytenoid  has  a 
distinct  center  of  chondrification,  while  the  cartilage  of  the  cricoid 
appears  as  a  single  ring  which  is  at  first  open  dorsally  and  only  later 
becomes  complete.  The  epiglottis  cartilage  resembles  the  thyreoid 
in  being  formed  by  the  fusion  o^  two  originally  distinct  cartilages, 
from  each  of  which  a  portion  separates  to  form  the  cuneiform 
cartilages  (cartilages  of  Wrisberg)  which  produce  the  tubercles  of 
the  same  name  on  the  ary-epiglottic  fold,  while  the  comiculate 
cartilitges  (cartilages  ofSantorint)  are  formed  by  the  separation  of  a 
small  portion  of  cartilage  from  each  arytenoid. 

The  formation  of  the  thyreoid  cartilage  by  the  fusion  of  two  pairs 
of  lateral  elements  finds  an  explanation  from  the  study  of  the 
comparative  anatomy  of  the  larynx.  In  the  lowest  group  of  the 
mammalia,  the  Monotremata,  the  four  cartilages  do  not  fuse 
together  and  are  very  evidently  serially  homologous  with  the  (ar- 
tilages  which  form  the  comua  of  the  hyoid.  In  other  words,  the 
thyreoid  results  from  the  fusion  of  the  fourth  and  fifth  branchial 
cartilages.  The  cricoid,  in  its  development,  presents  such  striking 
similarities  to  the  cartilaginous  rings  of  the  trachea  that  it  is  probably 
to  be  regarded  as  the  uppermost  cartilage  of  that  series,  but  the 
epiglottis  seems  to  be  a  secondary  chondrification  in  the  glosso- 
laryngeal  fold  (Schaffer).  The  arytenoids  possibly  represent  an 
additional  pair  of  branchial  cartilages,  such  as  occtir  in  the  lower 
vertebrates  (Gegenbaur). 

These  last  arches  have  undergone  almost  complete  reduction  in 
the  mammalia,  the  cartilages  being  their  only  representatives,  but, 


LITEKATCSE 


337 


in  addition  to  the  cartilages,  the  fourth  and  fifth  arches  have  also 
preserved  a  portion  of  their  musculature,  part  of  which  becomes 
transformed  into  the  muscles  of  the  larynx.  Since  the  nerve  which 
corresponds  to  these  arches  is  the  vagus,  the  supply  of  the  larynx  is 
derived  from  that  nerve,  the  superior  laryngeal  nerve  probably 
corresponding  to  the  fourth  arch,  while  the  inferior  (recurrent) 
answers  to  the  fifth. 

The  course  of  the  recurrent  nerve  finds  its  explanation  in  the  relation 
of  the  nerve  to  the  fourth  branchial  artery.  When  the  heart  occupies 
Its  primary  position  ventral  to  the  floor  of  the  pharynx,  the  inferior 
laryngeal  nerve  passes  transversely  inward  to  the  larynx  beneath  the 
fourth  branchial  artery.  As  the  heart  recedes  the  nerve  is  caught  by  the 
vessel  and  IS  earned  back  with  it,  the  portion  of  the  vagus  between  it  and 
the  superior  laryngeal  nerve  elongating  until  the  origins  of  the  two 
iaiyngea^  nerves  are  separated  by  the  entire  length  of  the  neck.  Hence  it 
IS  that  the  right  recu -rent  nerve  bends  upward  behind  the  right  subclav- 
aa  artery,  while  the  left  curves  beneath  the  arch  of  the  aorto  (see 
Fig.  149).  '^ 


LITERATURE. 

J.  M.  Flmt:  "TIk  Devdopmmt  of  the  Lungs,"  Amer.  Joum.  Anai.,  vi  1906 
J.  E.  F«AZX«:  "The  Devdopment  of  the  Laiyni,"  Jom,.  AwU.  md  Phys.,  XLIV,  loio. 
E.  GOPPMI:  ■  Ueber  die  Herkunft  der  Wrisbergschen  KnorpeU,"  Martha.  Jakrimck 
3oa,  1894.  ' 

W.  tts:  "Zur  BUdungsgeschkhte  des  Lungen  beim  memchUchcn  Embreo,"  Arclm 

far  Amia.  md  Pkysiol.,  Aiut.  Ablk.,  1887. 
E.  K«ina:  "BeitrSge  lur  Entwfckelungsgeschichte  da  Kehlkopfes,"  Anat.  Ht^ie, 

E.  Kaluds:  "Dk  Entwickelung  d«  menschlichen  Kehlkopfes,"  Verkmdl.  der  AmU 

Gttdbck.,  xn,  1898. 
A.  Lissn:  "Sludie.  on  the  Development  of  the  Human  Lmviii,"  Amtr.  Jaym 

Anat.,  xn,  191 1. 

A.  Namih:  "Der  Btonchialbaum  der  Singethiere  und  des  Menschen,"  Biblicthaa 

tltdtca,  Ablk.  A,  Heft  3,  1901. 
J.  S^Alrax:  "Zur  Hiitologie  Histogenese  und  phylogenetischen  Bedeutung  der 

Epigbtd.,"  Amu.  Htfit,  xxxm,  1907. 
A.  SoDLit  AND  E.  BAU>m:  "Recherches  sur  le  diveloppement  du  Uiyni  chez 

1  homme,"  Janm.  it  FAnat.  it  it  la  Physiol.,  xini,  1907. 


if 


ii- 


CHAPTER  XIII. 

THE  DEVELOPMEITT  OF  THE  URINOGENITAL  SYSTEM. 

The  excretory  and  reproductive  systems  of  organs  are  so  closely 
related  in  their  development  that  they  must  be  considered  together. 
They  both  owe  their  origin  to  the  mesoderm  which  constitutes  the 
intermediate  cell-mass  (p.  77),  this,  at  an  early  period  of  develop- 
ment, becoming  thickened  so  as  to  form  a  ridge  projecting  into  the 
dorsal  portion  of  the  coelom  ^d  forming  what  is  known  as  the 
Wolffian  ridge  (Fig.  207,  wr).    The  greater  portion  of  the  substance 


Fio.  107.— TiUNSviBsi  Section  ibkocoh  thx  Abdownal  Rxoion  or  a  Rabbit 

Ehbxyo  or  u  HH. 

•,  Aon*;  gl.,  glonuniliu;  p,  genital  ridge;  m,  mesentenr;  «,  notochord;  (,  tubule  of 

mesoneplnos;  wd,  WoffBui  duct;  wr.  Wolffian  ridge.— (ifttiirtovia.) 

of  this  ridge' is  concerned  in  the  development  of  the  primary  and 
secondary  excretory  organs,  but  on  its  mesial  surface  a  second  ridge 
appears  which  is  destined  to  give  rise  to  the  ovary  or  testis,  and 
hence  is  termed  the  genital  ridge  (gr). 

The  development  of  the  excretory  organs  is  remarkable  in  thai 
three  sets  of  oi|rans  appear  in  succession.    The  first  of  these,  the 
pronefkros,  exists  only  in  a  rudimentary  condition  in  the  human 
338 


TBK  FSONEFHSOS 


339 


embryo,  ahhough  its  duct,  the  pronephric  or  Wolffian  duct,  undergoes 
complete  development  and  plays  an  important  part  in  the  develop- 
ment of  the  succeeding  organs  of  excrerion  and  also  in  that  of  the 
reproductive  organs.  The  second  set,  the  mesonephro,  or  Wolffian 
body  reaches  a  considerable  development  during  embryonic  life, 
but  later,  on  the  development  of  the  final  set,  the  definite  kidney  or 
metanephros.  undergoes  degeneration,  portions  only  persisting  as 
rudimentary  structures  associated  for  the  most  part  with  the  repro- 
ductive organs.  ^ 

The  DeTelopment  of  the  Pronephros  and  the  Pronephric 
Duct.-The  first  portions  of  the  excretory  system  to  make  their 
appearance  are  the  pronephric  or  Wolffian  ducts,  which  develop  as 


Of,  Endoderm;  im,  intermeduite  cell  mua-  «.  m.«.j • 

outgrowthsof thedorsalwallsof theintermediatecell masses.  Atfirst 
he  outgrowths  are  solid  cords  of  cells  (Fig.  .08,  v,d),  but  later  a 

lumen  appears  in  the  center  of  each  and  the  canll  so  om^  from 
ach  mt^mediate  cell  mass,  bending  backward  at  its  free  eXoZ 

Wnt  Two  longitudinal  canals,  the  pronephric  or  Wolffian  ducts 
arethusformed,withwhichthecavitiesofthei„termediatecenmass2 
segments  before  the  segmentation  of  the  posterior  portions  of  the 

tubi  a '^"^  i;  K  '"'^'P''°''«°"y  of  the  formation  of  excretory 
tubules,  apparently  by  a  process  of  terminal  growth.    The  free  end 


340 


THE  FKOMXrHSOS 


of  each  duct  comes  into  intimate  relation  with  the  ectoderm  above  it, 
so  much  so  that  its  posterior  portion  has  been  held  by  some  observers 
to  be  formed  from  that  layer,  but  it  seems  more  probable  that  the 
relation  to  the  ectoderm  is  a  secondary  process  and  that  the  ducts 
are  entirely  of  mesodermal  origin.  They  reach  the  cloaca  in  em- 
bryos of  a  little  over  4  mm.,  and  later  they  unite  with  that  organ,  so 
that  their  lumina  open  into  its  cavity. 

The  pronephric  tubules  make  their  appearance  in  embryos  of 

I  about  1.7  mm.,  while  as  yet  there  are  only  nine  or  ten  mesodermic 

somites,  and  they  are  formed  from  the  intermediate  cell  masses  of  the 

seventh  to  the  fourteenth  segment,  and  perhaps  from  those  iituated 


W 
If 


Flo.  209.— DUOllAll  SHOWING  THE  STETJCTOlffi  01  A  FULLV  DlVElOPID 
PSONEFHUC  TUBtFLE. 

Ao,  Aorta;  Cue,  axiom;  «,  Ectodeim;  eg,  eztemal  gkjmerulus;  m,  eadoderm;  itt, 
m««odonmc  somite;  N,  nervous  system;  n,  nephmslDme;  nc,  nolDChord;  f.  pronepnnc 
dumber;  Wd,  Wolffian  duct.— (Modified  from  Ftlix.) 

Still  more  anteriorly.  The  entire  series,  however,  is  never  in  exist- 
ence at  any  one  time,  for  before  the  more  posterior  tubules  are 
formed,  those  of  the  anteri>  r  segments  have  undergone  degeneration. 
Each  pronephric  tubule,  when  fully  formed,  consists  of  a  portion 
which  unites  it  to  the  Wolffian  duct,  and  opens  at  its  other  end  into 
an  enlargement,  the  pronephric  chamber,  (Fig.  209,  pc),  which,  on 
its  part  opens  into  the  coelomic  cavity  by  means  of  a  nephrostome 
Canal.  In  the  neighborhood  of  the  coelomic  opening,  or  nephrostome, 
an  outgrowth  of  the  coelomic  epithelium  is  formed,  and  a  branch 
from  the  aorta  penetrates  into  this  to  form  a  stalked  external  glomer- 
ulus lying  free  in  the  coelomic  cavity  (Fig.   209,  e.g.).    Internal 


THE  ItESOMXFSROS 


341 


glomeruli,  such  as  occur  in  connection  with  the  mesonephric  tubules 
do  not  occur  in  the  pronephros  of  the  human  embryo,  and  this  fact, 
together  with  the  presence  of  external  glomeruli  and  the  participa- 
tion of  the  tubules  in  the  formation  of  the  Wolffian  duct,  serve  to 
distinguish  the  pronephros  from  the  mesonephros. 

The  pronephric  tubules,  are,  as  has  been  stated,  transitory 
structures  and  by  the  time  the  embryo  has  reached  a  length  of  about 
S  mm.  they  have  all  disappeared.    Before  their  disappearance  is 
complete,  however,  a  second  series  of  tubules  has  commenced  to 
develop,  forming  what  is  termed  the  mesonephros  or  Wolffian  body. 
The  Development  of  the  Mesonephros.— The  pronephric 
duct  does  not  disappear  with  the  degeneration  of  the  pronephric 
tubules,  but  persists  to  serve  as 
the  duct  for  the  mesonephros  and 
to  play  an  important  part  in  the 
development  of  the  metanephros 
also.     In  the  Wolffian  ridge  there 
appear  in  embryos  of  between  3 
and  4  mm.  a  number  of  coiled 
tubules,  which  arise  by  some  of 
the  cells  of  the  ridge  aggregating 
to  form  soUd  cords,  at  first  en- 
tirely unconnected  with  either  the 
coelomic     epithelium     or     the 
Wolffian  duct.    Later  the  cords 
become  connected  with  the  coe- 
lomic epithelium  and  acquire  a 
lumen,  and  near  the  coelomic  end 
of  the  tubule,  at  a  region  corresponding  to  the  chamber  of  a  pro- 
nephric lubule,  a  condensation  of  the  mesenchyme  of  the  Wolffian 
ridge  occurs  to  form  a  glomerulus  into  which  a  branch  extends  from 
the  neighboring  aorta.    The  tubule^l'IWl5r  acquire  conpection  with 
the  Wolffian  duct  and  at  the  same  time  lose  their  connections  with 
the  coelomic  epithelium,  their  nephrostomes  being  accordingly  but 
traiaitory  structures.    The  tubules  rapidly  increase  in  length  and 


Fig.  a  10. — Transverse  Section  of 
THE  Wolffian  Ridoe  of'a  Chick  Em- 
bryo OF  Three  Days? 

no,  AorU;  gl,  glomerulus;  gr,  genital 
ridge;  mes,  mesentery;  ml,  mesonephric 
tubule;  ve,  cardinal  vein;  Wd,  Wolffian 
duct. — (Mitutkovica) 


34* 


THZ  MB8CNXIBX0S 


become  coUed,  and  the  glomeruli  project  into  thar  cavities,  pushing 
in  front  of  them  the  wall  of  the  tubule  so  that  it  has  the  appearance 
represented  in  Fig.  aio. 

In  its  anterior  portion  the  Wolffian  ridge  is  formed  by  distinct 
intermediate  ceU  masses,  but  posterior  to  the  tenth  segment  it 
becomes  distinguishable  from  the  rest  of  the  mesoderm  before  this 
has  become  segmented,  and,  idling  to  undergo  transverse  division 
into  segments,  it  forms  a  continuous  column  of  cells,  known  as  the 
nephrogenic  cord.    The  anterior  tubules  of  the  mesonephros  make 
their  appearance  in  the  intermediate  cell  masses  belonging  to  the 
sixth  cervical  segment,  its  tubules  thus  overiapping  those  of  the 
pronephros,  and  from  this  level  they  appear  in  all  succeeding  seg- 
ments and  in  the  nephrogenic  cord  as  far  back  as  the  repon  of  the 
third  or  fourth  lumbar  segment,  where  the  cord  is  partially  inter- 
rupted.   This  interruption  marks  the  dividing  line  between  the  meso- 
nephric  and  metanephric  portio»s  of  the  cord,  the  portions  posterior 
to  it  being  destined  to  give  rise  to  the  metanephros.    But,  as  is  the 
case  with  the  pronephros,  the  entire  series  of  mesonephric  tubules  is 
never  in  existence  at  any  one  time,  a  degeneration  of  the  anterior 
ones  supervening  even  before  the  posterior  ones  have  differentiated, 
and  the  degeneration  proceeds  to  such  an  extent  that  in  an  embryo 
of  about  21  mm.  all  the  tubules  of  the  cervical  and  thoracic  segments 
have  disappeared,  only  those  of  the  lumbar  segments  persisting. 
This  does  not  mean,  however,  that  the  number  of  persisting 
tubules  corresponds  with  that  of  the  segments  in  which  they  occur,  for 
the  tubules  are  not  segmental  in  their  arrangement,  but  are  much 
more  numerous  than  such  an  arrangement  would  allow.    Two, 
three,  or  even  as  many  as  mne  may  correspond  with  the  extent  of 
a  mesodermic  somite  and  when  the  reduction  is  complete  in  an  embryo 
of  21  mm.,  where  only  the  tubules  corresponding  with  four  or  five 
s^ments  remain,  they  may  number  twenty-six  in  each  mesonephros 
(Felix).    This  arrangement  of  the  tubulei  together  with  the  size 
which  they  assume  when  fully  developed  brings  it  about  that  the 
Wolffian  ridges  become  somewhat  voluminous  structures  in  their 
mesonephric  portions,  projecting  markedly  into  the  cadomic  cavity 


THE   METANEFHX08 


343 


(Fig.  an).  Each  is  attached  to  the  dorsal  wall  of  the  body  by  a  dis- 
tinct mesentery  and  has  in  its  lateral  portion,  embedded  in  its 
substance,  the  WoIfiSan  duct,  while  on  its  mesial  surface  anteriorly 
is  the  but  slightly  developed  genital  ridge  (/).  This  condition  is 
reached  in  the  human  embryo  at  about  the  sixth  or  seventh  week  of 
development,  and  after  that  period  the  mesonephros  again  begins  to 
undergo  rapid  degeneration,  so  that  at  about  the  sixteenth  week 


F:o.  >ii.— UuMOOENtrAL  Appaiatds  or  A  Mali  Pio  Embryo  of  6  ch. 

00,  Aortt;  b,  bladder;  gh,  gubeiraculum  testis;  *,  kidney;  md,  MoUerian  duct;  sr 

suprarenal  body;  (,  testis;  w,  Wolffian  body;  wd,  Wolffian  duct.— (iWflWfcwia.) 

nothing  remains  of  ii  except  the  duct  and  a  few  small  rudiments 
whose  hktory  will  be  given  later. 

The  DeTeloimeiit  of  the  Hetanephros.— The  first  indication 
of  the  metanephros  or  permanent  kidney  is  a  tubular  outgrowth 
from  the  dorsal  surface  of  the  Wolffian  duct  shortly  before  its 
entrance  into  the  cloaca  (Fig.  170).  When  first  formed  this  out- 
growth lies  lateral  to  the  posterior  portion  of  the  Wolffian  ridge, 


344 


IHB  UIUWfBXOa 


which,  as  has  already  been  noted  (p.  344)1  i«  separated  from  the 
portion  that  gives  rise  to  the  meaonephios.  This  terminal  portion  of 
the  ridge  forms  what  is  termed  the  meUmetihrk  blastema  and  in 
embryos  of  7  mm.  it  has  come  into  relation  with  the  outgrowth  from 
the  Wolffian  duct  and  covers  its  free  extremity  as  a  cap.  Sinpe 
both  the  blastema  and  the  outgrowth  from  the  Wolffian  duct  take 

part  in  the  formation  of  the 
urinif erous  tubules,  these  have 
a  double  origin. 

The  outgrowth  from  the 
Wolffian  duct  as  it  continues  to 
elongate  comes  to  lie  dorsal  to 
the  mesonephros,  carrying  the 
cap  of  blastema  with  it,  and 
it  soon  assumes  a  somewhat 
club-shaped  form,  its  terminal 
enlargement  or  ampulla  form- 
ing what  may  be  termed  the 
primary  ratal  pelvis,  while  the 
remainder  represents  Aeureter. 
The  primary  renal  pelvis  then  gives  rise  to  from  three  to  six,  usually 
four,  tubular  outgrowths,  which  may  be  termed  primary  ctlUaiHg 
tubuUs,  and  with  their  formation  the  original  cap  of  metanephric 
blastema  undergoes  a  division  into  as  many  portions  ar<  there  are 
tubules,  so  that  each  of  the  latter  has  its  own  cap  of  blastema. 
As  soon  as  each  tubule  has  reached  a  certain  length  it  begins  to 
enlarge  at  its  free  extremity  to  form  an  ampulla,  just  as  did  the 
primary  renal  pelvis,  and  from  this  ampulla  there  grow  out  from 
two  to  four  seamdary  collecting  tubules,  a  further  corresponding 
division  of  the  metanephric  blastema  taking  place.  In  their  turn 
these  secondary  tubules  similariy  enlarge  at  their  extremities  to 
form  ampullse  (Fig.  212,  A)  from  which  tertiary  collecting  tubules 
are  budded  out,  accompanied  by  a  thurd  fragmentation  of  the  blastema 
and  so  the  process  goes  on  until  about  the  fifth  fetal  month,  the 
number  of  generations  of  collecting  tubules  formed  being  between 


Fio.  3ia. — DugiahsoiEauy  Snuos  IN 
thx  dlvsloplcent  of  thx  mxtahx^iuc 
Tubules. 

t,  Urinaiy  tubule;  Ur,  ureter;  v,  xtati  un- 
poll*.— (Haycrii/il.) 


TBI  IBIAmiBKOa 


34S 


eleven  and  thirteen,  each  tubule  of  the  final  generation  having  its 
cap  of  blastema. 

In  this  way  there  fa  formed  a  complicated  branching  system  of 
tubules  all  of  which  ultimately  communicate  with  the  primary 
renal  pelvis,  and  all  of  which  have,  in  the  last  analysfa,  liad  their 
origin  from  the  WolflMn  duct.  They  represent,  however,  only  the 
collecting  portions  of  the  uriniferous  tubules,  their  excreting  por- 


Fio.  J I  J. — Foot  Stages  in  ihi  Divelofunt  or  a  Uumnxous  TuBUtE  or  a  Cat. 
A,  Aiched  coUectiog  tubule,  C,  distal  convoluted  tubule;  C,  proximal  convoluted 
tubule;  H,  loop  of  Henle;  M,  glomenilua;  T,  renal  vesicle;  V,  ampulla  (drawn  from 
reonstiuctions  prepared  by  G.  C.  Huber). 

tions  having  yet  to  form,  and  these  take  their  origin  from  the  meta- 
nephric  blastema. 

When  the  terminal  collecting  tubules  have  been  formed  the 
blastemic  cap  in  connection  with  each  one  condenses  to  form  a  renal 
vesicle  (Fig.  213,  A,  T),  which  is  at  first  solid,  but  later  becomes 
hollow  and  proceeds  to  elongate  to  an  S-shaped  tubule,  one  end  of 
which  becomes  continuous  with  the  neighboring  ampulla  (Figs. 
212,  B,  and  213,  B),  and  in  the  space  enclosed  by  what  may  be 
tenned  the  lower  lo«^  of  the  S  a  collection  of  mesenchyme  cells 


[■•! 


34^  THE   HKTAMCIRBOt 

appean,  into  which  branches  penetrate  at  an  early  stage  from  the 
renal  artery  to  form  a  glomerulus,  the  neighboring  walls  of  the 
tubule  becoming  exceedingly  thin  and  being  transformed  into  a 
capsule  of  Bowman.  The  upper  loop  of  the  S  now  begins  to  elon- 
gate (Fig.  a  13,  C),  growing  toward  the  hilus  of  the  kidney,  parallel 
to  the  branch  of  the  outgrowth  from  the  Wolffian  duct  to  which  it  is 
attached  and  between  this  and  the  glomerulus,  and  forms  a  loop  of 
Henle.  From  the  portion  of  the  horizontal  limb  of  the  S  which  lies 
between  the  glomerulus  and  the  descending  limb  of  the  loop  of 
Henle  the  proximal  convoluted  tubule  (C)  arises,  while  the  distal 
convoluted  and  the  arched  collecting  tubules  (C  and  A)  are  formed 
from  the  uppermost  portion  of  the  upper  loop  (Fig.  213,  D).  The 
entire  length  of  each  uriniferous  tubule  from  Bowman's  capsule  to 
the  arched  collecting  tubule  inclusive  is  thus  derived  from  a  renal 
vesicle,  that  is  to  say,  from  the  metanephric  blastema. 

I 
Since  the  tubules  of  the  kidney  are  formed  by  the  union  of  two  originally 
distinct  structures  it  is  conceivable  that  in  the  cases  of  certain  tubules 
there  may  be  a  faUure  of  the  union.  The  blastemic  portion  of  tlie  tubules 
would,  nevertheless,  continue  tlieir  development  and  become  functional 
and,  since  there  would  be  not  means  of  esc^>e  for  the  secretion,  the  result 
would  be  a  cystic  kidney.  Occasionally  the  two  blaslemaU  of  opposite 
sides  fuse  across  the  middle  line,  the  result  being  the  formation  of  a 
single  transverse  or  horse-shoe  shaped  kidney,  or,  lAut  is  much  rarer,  the 
blastema  of  one  side  may  cross  the  middle  line  to  fuse  with  that  of  the 
other,  the  result  being  an  apparently  single  kidney  with  two  ureters  which 
open  normally  into  the  bladder. 

The  primary  renal  pelvis  is  the  first  formed  ampulla  and  does  not 
exactly  represent  the  definitive  pelvis.  This  is  produced  partly  by 
the  enlargement  of  the  primary  pelvis  and  partly  by  the  enlargement 
of  the  collecting  tubules  of  the  first  four  generations,  those  of  the  third 
and  fourth  generations  later  being  taken  up  or  absorbed  into  those 
of  the  seco:.,-^  generation,  so  that  the  tubules  of  the  fifth  generation 
appear  to  open  directly  into  those  of  the  second,  which  form  the 
calices  minores,  while  those  of  the  first  constitute  the  calices  majores. 
In  some  kidneys  the  process  of  reduction  of  the  earlier  formed 
collecting  tubules  proceeds  a  step  further,  those  of  the  first  generation 


THE   MtflLXUAN  DUCT 


347 


being  taken  up  into  tl>e  primary  renal  pelvis,  the  secondaries  then 
'orming  a  series  of  short  calices  arising  from  a  single  pelvic  cavity. 
At  about  the  tenth  week  of  development  the  surface  of  the  human 
kidney  becomes  marked  by  shallow  depressions  into  lobes,  of  which 
there  are  about  eighteen,  one  corresponding  to  each  of  the  groups 
of  tubules  which  arise  from  the  same  renal  vesicle.  This  lobation 
persists  until  after  birth  and  then  disappears  completely,  the  surface 
of  the  kidney  becoming  smooth. 

The  Development  of  the  Mttllerian  Duct  and  of  the  Genital 
Ridge.— At  the  time  when  the  WolflSan  body  has  almost  reached 
its  greatest  development  t>.s  WolflSan  ridge  is  distinctly  divided  into 
three  portions  (Fig.  214),  a  median  or  mesonephric  portion  attached 
to  the  body  wall,  a  lateral  or  tubal  portion  containing  the  Wolffian 
duct  and  attached  to  the  mesonephric  portion,  and  a  genital  portion, 
formed  by  the  genital  ridge  and  also  attached  to  the  mesonephric 
portion,  but  to  its  medial  surface.  In  the  tubal  portion  a  second 
longitudinal  duct,  known  as  the  MuUerian  dnct  (Fig.  314,  MS), 
makes  its  appearance.  Near  the  anterior  end  of  each  Wolffian 
ridge  there  is  formed  on  the  free  edge  of  the  tubal  portion  an  invag- 
ination of  the  peritoneal  covering,  and  by  the  proliferation  of  the 
cells  at  its  tip  this  invagination  gradually  extends  backward  in  the 
substance  of  the  tubal  portion  and  ieache3  the  cloaca  in  embryos  of 
about  33  mm.  The  primary  peritoneal  invagination  becomes  the 
abdominal  ostium  of  the  Mttllerian  duct,  the  backward  prolongation 
forming  the  duct  itself. 

In  Fig.  214  it  will  1)6  seen  that  the  tubal  portion  of  the  left 
Wolffian  ridge  is  somewhat  bent  inward  toward  the  median  line 
and  in  the  lower  parts  of  their  extent  this  becomes  more  pronounced 
in  both  tubal  portions  until  finally  their  free  edges  come  in  com  .ct 
and  fuse  in  the  median  line,  while  at  the  same  time  their  lower  edges 
fuse  with  the  floor  of  the  coelomic  cavity.  In  this  way  a  transverse 
partition  is  formed  across  what  will  eventually  be  the  pelvis  of  the 
adult,  this  cavity  being  thus  divided  into  two  compartments,  a 
posterior  one  containing  the  lower  portion  of  the  intestine  and  an  ante- 
rior one  containing  the  bladder.    With  the  formation  of  this  trans- 


34« 


TBI  aCNITAL  UDOB 


Fio.  at4.-T«AMSvi»SE  SicnoN  TmioooH  rai  ABDomuia  Rkuon  of  an  Emivo 

or  35  IOC. 
.„i-'*f'  *'?*i-^'  '''"'''"■i{.  intestine;  i,  Uver;  M,  muscle;  Md,  MflUerian  duct-  N 
spjnal  cord;  Or,  ovary;  RA,  rectus  abA>minis;  Sg.  spinal  gan^lion;^  uJSJiUc^ 
•rterjr;  :/r,  ureter;  V,  vertebra;  W,  WolSan  body; V^T Wol&^duct -(*^rf) 


IBB  OBNITAL  IIDGE 


349 


m.efold.  which  is  represented  by  the  b«>ad  ligament  in  the  female. 
^MoUenan  duct,  of  opposite  .ide.  are  brought  into  contact  «.d 

WoiaTrit  '"•"'V"^'"  °f  'he  mesonephric  portion  of  the 
Wolffian  ndge  a  longjtud.nal  elevation  i,  f.rmcd  at  about  this  time. 

theflli  Iffh  /  *"'•  ""  '••*  ""'""  '  ''"^  ^^"^-^^  f°W  with 
he  floor  of  thecoelom,c  cavity  it  comesl.u,,  ..iHct...„l  f,..o,  with  the 
tower  par.  of  the  anterior  abdomina'  ■••..;.  ,u:  f  i  ..cal ,.  ,  .e  lateral 
^nler  of  the  rectus  abdominis  n,..  .,.  i„  „..  „,„„„,  J^ 
fold  the  mesenchyme  condenses  to  for,.-  ..  licnrnv;-  .  ord  fh» 
mp«j^/^.^  whose  further  h.tory  v.ll  ...  consxind  lat^ron 
nf  .h  *!r^  "  *'  "'''**  itsape.ar.nr  ..„  .  >,,„^.,ite  thickening 
o<  the  epithelium  covenng  the  mesial  surface  of  .'u  W  .Iffian  ridge 
(Fig.  207  «r)  Utercolumns  of  cells  gro.  Cr-n  fron.  the  thicken- 
n?  S     ?         '*  °^  "^  ^"'^"  "■•'8'='  '*"P'*='"«  «>»«  '»««'neph- 

LS   f  r  Vf^''\  ""  ''^''  "♦*°'-    '^^'^  ^°""»«  a«  com- 
posed of  two  kinds  of  cells:  (,)  smaUer  epiAelial  ceUs  with  a  reU- 

11'  r VT""'/  r'""'"  ■""*  (^)  '"»'^'  ^P'x'rical  cells  whh 

growth  of  the  cell-column,  down  into  the  substance  of  the  Wolffian 

Uons  of  the  length  of  the  genital  ridge.  Indeed,  three  regfens 
may  be  recognaed  in  the  ridge;  an  anterior  one  in  which  a  relativelv 
smll  number  of  ceU-columns,  extending  deeply  into  the  sS^  s 
formed;  a  middle  one  m  which  numerous  columns  are  formed;  and 
a  posterior  one  m  which  practicaUy  none  are  formed.  The  first 
r^on  has  been  termed  the  rete  region  and  its  ceU-coIumns  the  rete- 

^r'nrT  T.^  '"^°°  """  "^'^^  "^  ""^  its  Columns  the 
s«^cords  and  the  posterior  region  is  the  mesmUric  region  and  plays 
no  part  m  the  actual  formation  of  the  ovary  or  testis 

appearance  in  the  endoderm  of  thi  Z^T^^  TSenc^'th'^^a?^' 


35° 


THE   TESTIS 


into  the  mewnteiy  and  lome  of  them  eventually  into  the  peritoneum 
covering  the  mesial  surface  of  the  Wolffian  ridge,  where  they  give  rise 
to  the  sex-cells  found  in  the  epithelium  of  the  genital  ridge.  Thu  origin 
of  the  sex-cells  has  not  yet  been  observed  in  the  human  embryo. 

The  various  steps  in  the  differentiation  of  the  reproductive 
oi;gans  so  far  described  occur  in  all  embryos,  no  matter  what  their 
future  sex  may  be.  The  later  stages,  however,  differ  according  to 
sex,  and  consequently  it  will  be  necessary  to  follow  the  further 
development  first  of  the  testis  and  then  of  the  ovary,  the  changes 


Fio.  ai5. — SicnoN  THXonoH  the  Txstb  and  thi  Bsoao  Lioakent  or  thk  Txstis 

OI  AH  EmiYO  01  5.5  HM. 

tp.  Epithelium:  md,  MaUerian  duct;  mo,  mesordliuin;  re,  rete-cords;  u,  aez-cords:  wd, 
WolSu  duct.— (VMoUinitci.) 

that  take  place  in  the  ducts  and  other  accessory  structures  being 
reserved  for  a  special  section. 

The  DevehpmeiU  of  the  Testis. — At  about  the  fourth  or  fifth  week 
there  appears  in  the  sex-gland  region  of  the  genital  ridge  a  structure 
which  serves  to  characterize  the  region  as  a  testis.  This  is  a  layer 
of  somewhat  dense  connective  tissue  which  grows  in  between  the 
epithelial  and  stroma  layers  of  the  sex-gland  region  and  gradually 
extends  around  almost  the  entire  sex-gland  to  form  the  tunica  albu- 
ginea.    By  its  development  the  sex-cords  are  separated  from  the 


THE  TEsns 


351 


epithelium,  which  later  becomes  muchlflattened  and  eventuaUy 
atoost  disappears.  Shortly  after  the  appearance  of  the  albuginea 
the  sex-cords  umte  to  from  a  complicated  network  and  the  rete-cords 
grow  backward  along  the  line  of  attachment  of  the  testis  to  the 
mesonephnc  portion  of  the  Wolffian  ridge,  coming  to  lie  in  the  hilus 


^^  '^l^r"™""  *"™*'' "  ™  ^""^  "  *"  Embvo  Cat  or  ,.4  cm. 

of  the  testis  (Fig.  215).  They  then  develop  a  lumen  and  send  off 
branches  which  comiect  with  the  sex-coid  reticulum  and  they  also 
make  connection  with  the  glomerular  portions  of  the  tubules  belong- 
mg  to  the  anterior  part  of  the  mesonephros.  Since  hke  the  sex- 
cords,  they  have  by  this  Ume  separated  from  the  epithelium  that 


3S» 


TBI   OVAXY 


gave  rise  to  them,  they  now  extend  between  the  sex-cord  reticuluip. 
and  the  anterior  mesonephric  tubules.  Certain  portions  of  the 
sex-cords  now  begin  to  break  down  leaving  other  portions  to  form 
convoluted  stems  which  eventually  become  the  seminiferous  lubuUs, 
while  from  the  rete-cords  a.e  formed  the  htbuU  recti  and  reie  testis, 
by  which  the  spermatozoa  are  transmitted  to  the  mesonephric 
tubules  and  so  to  the  Wolfifian  duct  (see  p.  355). 

The  development  of  the  seminiferous  tubules  is  not,  however, 
completed  until  puberty.  The  stems  derived  from  the  sex-cords 
form  cylindrical  cords,  between  which  lie  stroma  cells  and  in- 
terstitial cells  derived  from  the  stroma;  but  until  puberty  these  cordi 
remain  solid,  a  lumen  developing  only  at  that  period.  The  cords 
contain  the  same  forms  of  cells  as  were  described  as  occurring  in  the 
epithelium  of  the  germinal  ridge,  and  while  in  the  early  stages 
transitional  forms  seem  to  occur,  in  later  periods  the  two  varieties  of 
cells  are  quite  distinct,  the  sex-cells  becoming  spermatogonia 
(see  p.  14)  and  being  the  mother  cells  of  the  spermatozoa,  while  the 
remaining  epithelial  cells  perhaps  become  transformed  into  the  con- 
nective-tissue walls  of  the  tubules. 

The  Devehpment  of  the  Ovary.— In  the  case  of  the  ovary,  after 
the  formation  of  the  sex-cords,  connective  tissue  grows  in  between 
these  and  the  epithelium,  forming  a  layer  equivalent  to  the  tunica 
albuginea  of  the  testis.  It  is,  however,  a  much  looser  tissue  than 
its  homologue  in  the  male,  and,  indeed,  does  not  completely  isobwe 
the  sex-cords  from  the  epithelium,  although  the  majority  of  the  coi* 
are  separated  and  sink  into  the  deeper  ;>ortions  of  the  ovary  where 
they  form  what  have  been  termed  the  mediiUary  cords.  In  the  mean- 
time the  germinal  epithelium  has  continued  to  bud  off  cords  wtuch 
unite  to  form  a  cortical  layer  of  cells  lying  below  the  epithelium  and 
separated  from  the  medullary  cords  by  the  timica  albuginea 
(Fig.  216). 

Later  the  cortical  layer  becomes  broken  up  by  the  ingrowlSi  o' 
stroma  tissue  into  spherical  or  cord-like  maises,  consisting  of  sex 
cells  and  epithelial  cells  (Fig.  217).  The  invanioii  of  the  strom. 
continuing,  these  spheres  or  cords  (Pfiiiger's  cord.')  become  divide 


IHE    OVAXY 


353 


mto  smaUer  ina«es,  the  primary  ovarian /Mide,,  each  of  which 
co|»uU  as  a  rule  of  a  single  sex<eU  sur«>unded  by  a  number  of 

o™T.  r%:  'k  r"*"*"^  *^°'"«'  "='"y  vascularized  and 
forms  a  theca  folhcuh  (Fig.  ,o).  The  epithelial  cells  in  each  follicle 
are  at  fi«t  comparafvely  few  in  number  and  closely  sun«und   the 

r      ff  VI' ^'  ''^'^  '■*  ^'^^^  '»  1^"""=  »  ovum,  but  in 
cemm  of  the  foUicles  they  umleigo  an  increase  by  mitosis,  booming 
extremely  numerous,  and  later 
secrete  a  fluid,  the  Ikjuor  folli- 
culi,  which  coHects  at  one  side  of 
the  follicle  and  eventually  forms 
a  considerable  portion  of  its  con 
tents.    The  foilic-iUr  ceils  are 
differentiated  by  its  appearanrt 
into  the  stratum  gramlosum, 
which  surrounds  the  wall  of  the 
follicle,  and  the  discus  prMgerus, 
in  which  the  ovum  is  embedded 
Tig.  10,  dp),  and  the  ceUs  which 
immediately  surround  the  ovum, 
incoming  cylindrical   in  shape, 
give  rise  to  the  corona  radiata 
Fij.  II,  cr). 

A  somev^t  similar  fate  is 
^Lanri  by  the  medullary  cords,  these  also  Diking  up  into  a  num- 
-^  of  foUicles,  but  sooner  or  lah=r  these  toUicles  undergo  degmera- 
Non  so  that  shortly  after  birth  pracfcall.  «,  traces  o,  the  corts  re- 
main. It  must  be  noted  that  degeneranon  of  the  foUicxes  formed 
««>  the  cortical  layer  also  takes  place  even  duri.^  fetal  life  and 

'mimues  to  occur  throughout  the  entire  period  of  growth  and  func- 
1'^^  actmty,  numer«.s  atretic  tniicles  beiag  found  in  the  ovary 
''  *fl  times.  Indeed  it  wo«ld  sear  tha  deRmeration  U  the  fate  of 
>' '■  sreat  majority  of  the  follicles  ami  s«-celt  »i  the  ovary  but  few 

a  commg  to  maturity  during  t»-  lijt-time  at  any  individual 


Fbs.  2i7.-ci«cTiON  OF  TBI  Ovary  of 

A  HIW-BO^  CBLD. 

.J*"  2i!!?*'.  ''!**=l'on>^  »,  proiimiil  part 
•  i  ,V^  '•  'P**"^  nuMes;  /  pri- 
Gtfmtmm.  a^  WaUtyer.) 


354 


THE   GBNRAL  DUCT8 


Rete-cotds  developed  from  the  rete  portion  of  the  germinal 
ridge  occur  in  connection  with  the  ovary  as  well  as  with  the  testis 
and  form  a  rete  marii  (Fig.  216,  R).  They  do  not,  however,  ertend 
so  deeply  into  the  ovary,  remaining  in  Ae  neighborhood  of  the 
mesovarium,  and  they  do  not  become  tubular,  but  resemble  closely 
the  medullary  cords  with  which  they  are  serially  homologous. 
They  separate  from  the  epitheUum  and  make  connections  with  the 
glomeruli  of  the  anterior  portion  of  the  mesonephros,  on  the 
one  hand,  and  on  the  other  with  medullary  cords,  and  m  later 
stages  show  a  tendency  to  break  up  into  primary  follicles,  which 
early  dsgenerate  and  disappear  like  those  of  the  medullary  cords. 

The  Trantforaurtioii  of  the  Meione^os  and  the  Ducts.— 
At  one  period  of  devdojwient  there  are  present,  as  representatives 
of  the  urinogenital  apparatus,  the  WolfiSan  body  (mesonephros) 
and  duct,  the  Miillerian  duct,  and  the  developing  ovary  or  tesUs. 
Such  a  condition  forms  an  indifferent  stage  from  which  the  develop- 
ment proceeds  in  one  of  two  directions  according  as  the  goiital 
ridge  becomes  a  testis  or  an  ovary,  the  Wolffian  body  m  fmrt  under 
going  degeneration  and  in  part  persisting  to  form  organs  which  for 
the  most  part  are  rudimentary,  while  in  the  female  the  Wolftar 
duct  also  degenerates  except  for  certain  rudimentt  and  in  the  male 
the  MuUerian  duct  behaves  similarly. 

In  the  Male.-V  has  been  seen  that  the  Wolffian  body,  through 
the  rete  cords,  enters  into  very  intimate  relations  with  the  testis, 
and  it  may  be  regarded  as  divided  into  two  portioee,  an  apper 
genital  and  a  lower  excretory.  In  the  male  the  genittl  portion 
persists  in  its  entirety,  serving  as  the  efferent  ducts  of  thr  test.  . 
which,  beginning  in  the  spaces  of  the  rete  testis,  ahxady  shown  to  t>.' 
connected  with  the  capsules  of  Bowman,  open  into  the  upper  p«ft  of 
the  Wolffian  duct  and  form  the  globus  major  of  the  epididymis. 
The  excretory  portion  undergoes  extensive  degeneration,  a  partion 
of  it  persisting  as  a  mass  of  coiled  tubules  ending  blindly  at  boh 
ends,  situated  near  the  head  of  the  epididymis  and  known  as  t  * 
paradidymis  or  organ  of  Giraldis,  whilp  a  single  elongated  tubue. 
arising  from  the  portion  of  the  Wolffian  duct  which  fonm  i  e 


THE   GEDIIAL  DOCTS 


355 


globus  minor  of  the  epididymis,  represents  another  portion  of  it  and 
is  known  as  the  wis  aberrtms. 

The  WolflSan  duct  is  retained  complete,  the  portion  of  it  nearest 
the  testis  becoming  greatly  elongated  and  thrown  into  numerous 
coils,  forming  the  body  and  globus  minor  of  the  epididymis,  while  the 
remainder  of  it  is  converted  into  the  vas  deferens  and  the  ductus 
ejaculalorius.  A  lateral  outpouching  of  the  wall  of  the  duct  to 
form  a  longitudinal  fold  appears  at  about  the  third  month  and 
gives  rise  to  the  vesicula  semmalis,  the  lateral  position  of  the  out- 
growth explaining  the  adult  position  of  the  vesicuUe  lateral  to  the 
vasa  deferentia. 

With  the  MUlJenan  dticU  the  case  is  very  different,  since  they 
diMfltear  compietdr  throughout  the  greater  part  of  their  course, 
o^  Aeir  upper  and  Jnwer  ends  persisting,  the  former  giving  rise  to  a 
smtK-  mc-Uke  body,  tke  sewHe  kydadd  of  MorgagHi,  attached  to  the 
uppereadof  each  testis  neartheepididtwis.  It  has  been  seen  (p.349) 
•hat  li»e  tower  eMb  of  the  Mulieriaa  dues*,  in  the  male  as  well  as  the 
«male,  fuse  to  btem  the  utero-va^MJ  caaai,  and  the  lower  portion 
«»?  this  also  persirts  .0  form  whaK  le  termed  the  uterus  masculinus, 
attihough  It  correspomis  to  tke  vapu  o*  the  fcmaJe  rather  than  to  the' 
uterus.  It  is  a  sh«jrt  cslindiica!  peach  of  varying  length,  that  opens 
into  thf  urethra  at  the  bettom  of  a  depfessiou  known  aa  the  utriculus 
pro^aticux  (mms  poaiUms). 

The  transverse  pelvic  iiartition,  produced  by  the  union  of  the  two 
lubal  portioM  of  the  WoHian  body,  is  Mmned  in  the  male  embryo, 
i)ut  at  as  early  stage  its  aaterioi  s«face  fuses  with  the  posterior 
surface  of  the  bladder  and  coosequentlff  there  is  in  tlic  male  no  pelvic 
compartment  equivalent  to  the  vesico-uterine  pouch  of  the  female. 
The  male  reGlo--.e«ital  pouch  »,  howewr  lae  homologue  of  the  recto- 
uterine pouch  of  ihe  femait 

The  formstiwi  «^  the  ii^uinal  ligament  ^  the  surface  of  the 
nejonepiuos  sm  been  described  on  p.  J49.  On  the  degeneration  of 
ae  aesonephros  the  iayer  of  pemoDean  dwt  yjvered  it  persists  to 
i'MM  a  mestrchUtm  extending  from  the  body  wa«  ia  fh..  hilus  of  the 
irttii  and  the  inguinal  hgament  now  comes  to  have  its  origin  from 


30 


THE  osiimi-  sucn 


.J.,  l«»er  Dote  of  that  o.»«,  whence  it  extemis  to  the  anterior  ab- 
1^^^  0^<^^^  -»'—'»'>  nature  of  the  utem 
dommal  wa«  "»    .     development  of  its  waUs  the  inguinal 

si'^„rdo:lot^t:2lv^ed^i«.*---i-'^^^^ 

aSd  forms  the  gubcm<,culum  testis  of  the  adult,  whose  adult  ^i- 
^'„  SZughtlut  by  the  descent  of  the  testis  into  the  scrotum 

^%l'^Femuk-ln  the  female  the  transverse  partition  of  the 
pelv^  dts  nrtseVth  the  bladder  but  remains  distinct  as  the 
^ZmfS  Consequently  there  is  in  the  female  both  a  vesico- 
Sn  anTt'recto-uterine  ,x,uch.  Since  the  geniul  ridges  fomi 
uS.  thrm^al  surface,  of  the  Wolffian  ridges  and  the  ti^ 
;^oL  are  their  lateral  portion.,  when  these  Utter  umte^  ^°- 
Ae  broad  ligament  the  <.var,  wiU  come  to  he  upon  the  postenor 
^^T  of  tL  structure,  projecting  into  the  recto-vcsical  pouch 
^X  degeneration  of  the  mesonephros  the  peritoneum  toat 
°  veS  itl^omes  a  part  of  the  broad  ligament,  fonning  that  v^ 
TutwchTontains  t^Fallopian  tubes  and  hence  is  known  »s  the 
Jelollpi".  while  the  lower  part  of  the  Ugament,  on  account  of  its 
relation  to  the  uterus,  is  termed  the  mesometnum. 

The  lenital  portion  of  the  mesonephros,  though  never  functional 
as  d^L Tthe  female,  per«sts  as  a  group  of  ten  to  fifteen  tubule^ 
rit«"S  ^tween  the  two  layers  of  the  broad  ligament  and  m  do^ 
Bituatea  oeiwee  constitute  what  is  known  as  the 

proximity  to  the  o^*^'  *^'°    ^.^^afcr).    The  tubules  ter- 
etomhorou  (jxmvanum  or  organ  o]  Kosmmtuter). 
iS^UnS-rthe  ends  nearest  the  ovary,  but  at  the  oth^   x^ 
tomity  ^here  they  are  somewhat  coiled,  they  open  mto  a  coUecting 
S^nchrepresentstheupperendoftheWolffii«duct.    Near  t^ 

id  mentary  body  is  another,  also  composed  of  tubules,  representing 
r^SomLxcretory  portion  of  the  me«.neph™^^^^^^^^^^ 
the  parooplu>ro»  which,  however,  degenerates  during  he  earjy  year^ 
S  ex^-uterine  life.    So  far  as  the  mesonephros  is  co-.rned  there 
lor^^e  persisting  rudiments  in  the  female  are  comparable  to  tho. 

'^TCtZtc.,  however,  the  case  is  difiere»t,  for  in  th. 


THE  OENITAL  DUCTS 


357 


female  it  is  the  Mttllerian  ducts  which  persist,  while  the  Wolffians 
undeigo  degeneration,  a  small  portion  of  their  upper  ends  persisting 
in  connection  with  the  epo5phora,  while  their  lower  ends  persist  as 
straight  tubules  lying  at  the  sides  of  the  vagina  and  forming  what 
are  known  as  the  canals  of  Gartner.  The  MUllerian  ducts,  on  the 
other  hand,  become  converted  into  the  Fallopian  tubes  {luba  lOerma), 
and  in  their  lower  portions  into  the  uterus  and  vagina.  From  the 
margins  of  the  openings  by  which  the  MOllerian  ducts  communicate 
with  the  coelom  projections  devdop  at  an  early  period  and  give  rise 
to  the  fimbria,  with  the  exception  of  the  one  connected  with  tht 
ovary,  the  fimbria  ovarica,  which  is  the  persisting  upper  portion  of 
the  original  genital  ridge.  From  the  utero-vaginal  canal  the  two 
structures  which  give  it  its  name  are  formed,  the  entire  canal  being 
transformed  into  the  mucous  membrane  of  the  uterus  and  vagina. 
Indeed,  the  lower  ends  of  the  Fallopian  tubes  are  also  taken  up 
into  the  uterus,  for  the  condensation  of  mesenchyme  that  takes 
place  around  the  mucosa  to  form  the  muscular  wall  of  the  uterus 
is  so  voluminous  that  it  includes  not  only  the  utero-vaginal  canal 
but  also  the  adjacent  portions  of  the  Miillerian  ducts.  The  histo- 
logical differentiation  of  the  uterus  from  the  vagina  begins  to 
manifest  itself  at  about  the  third  month,  and  during  the  fourth 
month  the  vaginal  portion  of  the  duct  becomes  flatteaed  aad  the 
epithelium  lining  its  lumen  fuies  so  as  to  completely  ocdnde  it 
and,  a  little  later,  there  appcmrs  at  its  lower  opening  a  dotiact  semi- 
circular fold.  This  is  the  iymen,  a  structure  which  seems  to  be 
represented  in  the  male  by  the  caUiculms  seminalis.  The  obliteration 
of  the  lumen  of  the  vagina  persists  until  about  the  sixth  month, 
when  the  cavity  is  re-established  by  the  breaking  down  of  the  central 
epithelial  cells. 

The  extent  of  the  mesenchymal  condensation  to  form  the 
muscularis  uteri  also  produces  a  modification  of  the  relations  of  the 
inguinal  ligament  in  the  female.  For  the  ligament  becomes  for  a 
~hort  portion  of  its  length  included  in  the  condensation  and  thus 
attached  to  the  upper  portion  of  the  uterus.  It  ij  consequently 
divided  into  two  portions,  one  extending  from  the  lower  pole  of 


358 


THE   OENITAL  DUCTS 


the  ovary  to  the  utenu  and  fonning  the  ligamenium  ovarii  frofrkm 
and  the  other  eztendiog  from  the  uterus  to  the  anterior  abdominal 
wall  and  fo.  ming  what  is  known  in  the  adult  as  the  roitnd  UgameM 
of  the  uterus. 

The  diagram,  Fig.  ai8,  illustrates  the  transformation  from  the 
indifferent  condition  which  occurs  in  the  two  sexes,  and  that  the 


FlO    JI».— DiAORAllS  IlLnSTIATINO  TFK  T«ANSrO«MAnON  OF  THE  Mttl.««l*N  AND 
WOirFIAN  DOCTS. 

B  Bludder;  C,  clitora;  CG,  canil  of  G««rtner;  CI,  cloKa;  So,  epoOphonm;  Bp,'^}- 
(Sdrmis-  P.  F»Uopi«n  tube;  C,  goiial  gland;  HE,  hydatid  of  epididymis;  HU,  hydatid 
of  Morguni;  K,  Itidney;  UD.  MttUeriMi  duct;  O,  ovary;  P.  penia;  Po  ptAXXiphTOn;  fr. 
proatrtiTlaad;  R.  rectum;  T,  testis;  U.  urethra;  UU,  uterus  masculmus;  t/r,  ureter; 
US,  urogenital  simB.  Ul,  uterus;  V,  vagina;  Va,  vas  aberrana;  VD  vm  dtfetOTs;  Ki, 

•     ■  ■     ■■     Wfl,  WolSan  body;  IKD,  Wolffian  duct.— (Jlfo*^  A""  ff'M'f}') 


hcHDologies  of  the  various  parts  may  be  clearly  understood  they 
may  also  be  stated  ia  tabular  form  as  on  the  next  page. 


M 


THE   BLADDEK 


359 


In<U0crenl  SUgc. 


M*k. 


Genital  ridge 


WoUEtnbody. 


Totk. 
Gubenwctilum. 


Frmmle. 


Fimbria  ovarica. 
Ovary. 

Ovarian  ligament. 
Round  ligament. 


Globui  major  oi  epidldymit.     i  Epo5phoron. 
Paradidymja.  :  ParoOplioron. 

Vaia  aberrantla. 


Body   and   globui 

mlBor 

of 

Collecting    tubules   of   epo- 

epididymis. 

Aphoroo. 

Wolffian  dacta. ... 

/aaa  deterentia. 

Seminal  vesicles.' 

1  Ejaculalory  ducts. 

Canal  of  Gartner. 

Sesule  hyatid. 

Fallopian  tubes. 

Mallerianducte... 

Uterus. 

Uterus  maacuHnua. 

Vagta. 

In  addition  to  the  sessile  hydatid,  a  slallud  hydatid  also  occurs  in 
connection  with  the  testis,  and  a  siinilar  structure  is  attached  to  the 
fimbriated  opening  of  each  Fallopian  tube.  The  significance  of  these 
structures  is  uncertain,  though  it  has  been  suggested  that  they  are  per- 
sisting rudiments  of  the  pronephros. 

A  failure  of  the  developracnt  of  the  various  parts  just  described  to  be 
completed  in  the  normal  manner  leads  to  various  abnormalities  in  con- 
nection with  the  reproductive  organs.  Thus  there  may  occur  a  failure 
in  the  fusion  of  the  lower  portions  of  the  Milllerian  ducts,  a  bihomed  or 
bipartite  uterus  resulting,  or  the  two  ducts  may  come  into  contact  and 
their  adjacent  walls  fail  to  disappear,  the  result  being  a  median  partition 
separ:;iir.);  the  vagina  or  both  the  vagina  and  uterus  into  two  compart- 
ments. The  excessive  development  of  the  fold  which  gives  rise  to  the 
hymen  may  lead  to  a  complete  closure  of  the  lower  opening  of  the 
vagina,  while,  on  the  other  hand,  a  failure  of  the  Milllerian  ducts  to 
fuse  may  produce  a  biperforate  hymen. 

The  DvTelopment  of  the  Uriiury  Bladder  and  the  Uro- 
genital Sinus. — So  far  the  relations  of  the  lower  ends  of  the  urino- 
genital  ducts  have  not  been  considered  in  detail,  although  it  has  been 


36o 


1HK   BIADDEX 


seen  that  in  the  early  stages  of  development  the  WolflSan  and 
MUUerian  ducts  open  into  the  sides  of  the  ventral  portion  of  the 
cloaca;  that  the  ureters  communicate  with  the  lower  portions  of  the 
Wolffian  ducts;  that  from  the  ventral  anterior  portion  of  the  cloaca 
the  aUantoic  duct  extends  outward  into  the  belly-stalk;  and,  finally 
(p.  a8i),  that  the  cloaca  becomes  divided  into  a  dorsal  portion,  which 
forms  the  lower  part  of  the  rectum,  and  a  ventral  portion,  which  is 
continuous  with  the  allantois  and  receives  the  urinogenital  ducts 


FW.  119.— RxcomnncnoN  o»  the  Cloacal  Region  ot  am  Embeyo  01  14  HM- 

«/.  AUantois;  h,  bUdder;  gl,  geniul  tubwdei  i,  intaline;  «^»ptoal  cord:  «^tochord  ; 

r,  rectum;  »j,  uiogoatal  sinus;  w,  ureter;  w.  Wolffian  duct— (JCnSX.) 

(Fig.  aip).    It  is  the  history  of  this  ventral  portion  of  the  cloaca 
which  is  now  to  be  considered. 

It  may  be  regarded  as.  consisting  of  two  portions,  an  anterior  and 
a  posterior,  the  line  of  iiisertion  of  the  urinogenital  ducts  marking  the 
junction  of  the  two.  The  anterior  or  upper  portion  is  destined  to 
give  rise  to  the  urinary  bladder  (Fig.  219,  b),  while  the  lower  om 
forms  what  is  known  for  a  time  as  the  urogenUal  sinus  (sg).  The 
bladder,  when  first  differentiated,  is  a  tubular  structure,  whose 
lumen  is  continuous  with  that  of  the  allantois,  but  after  the  second 


TBI  BUDDKR  361 

month  It  enUi:ges  to  become  more  lac-like,  while  the  intra-embryonic 
portion  of  the  allantois  degenentes  to  a  lolld  cord  extending  from 
the  apex  of  the  bladder  to  the  umbilicus  and  is  known  as  the  wachus. 
During  the  enlargement  of  the  bladder  the  terminal  portions  of  the 
urinogenital  ducts  are  taken  up  into  iu  walb,  a  process  which 
continues  until  finally  the  ureters  and  Wolffian  ducts  open  into  it 
separately,  the  ureters  opening  to  the  sides  of  and  a  little  anterior 
to  the  ducts.    This  condition  is  reached  in  embryos  of  about  14  mm. 


Fio.  Jao.— RECONST»ucnoN  or  the  Cumcal  Shiuctotm  of  an  Eiibryo  ur  15  mt. 

«,  Bladder;  m,  MtlUerian  duct;  r,  rectum;  sg,  urogenital  sinus;  sy,  symphysis  pubis-  «, 

ureter;  wr,  urethra;  «r.  Wolffian  duct.— (i4ifa^R</riM  KeiM.) 

(Fig.  219),  and  in  later  stages  the  interval  between  the  two  pairs  of 
ducts  is  increased  (Fig.  220),  resulting  in  the  formation  of  a  short 
canal  connecting  the  lower  end  of  the  bladder  which  receives  the 
ureters  with  the  upper  end  of  the  urogenital  sinus,  into  which  the 
Wolffian  and  MUUerian  ducts  open.  This  connecting  canal  repre- 
sents the  urethra  (Fig.  220,  ur),  or  rather  the  entire  urethra  of  the 
female  and  the  proximal  part  of  that  of  the  male,  since  a  considerable 
portion  of  the  latter  canal  is  still  undeveloped  (see  p.  364).    Frx>m 


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362  THE   OTtOGENlTAL  SINUS 

this  urethra  there  is  developed,  at  about  the  third  month,  a  series  of 
solid  longitudinal  folds  which  project  upon  the  outer  surface  and 
separate  from  the  urethra  from  above  downward.  These  represent 
the  tubules  of  the  prostate  gUmd  and  are  developed  in  both  sexes, 
although  they  remain  in  a  somewhat  rudimentary  condition  in  the 
female.  The  muscular  tissue,  so  characteristic  of  the  gland  in  the 
adult  male,  is  developed  from  the  surrounding  mesenchyme  at  a 
later  stage. 

The  bladder  is,  accordingly,  essentially  a  derivative  of  the  cloaca 
and  its  mucous  membrane  is  therefore  largely  of  endodermal  origin. 
Portions  of  the  Wolffian  ducts  which  are  of  mesodermal  origin  are, 
however,  taken  up  into  the  wall  of  the  bladder  and  form  a  portion 
of  it.  The  extent  of  the  portion  so  formed  is  indicated  by  the 
position  of  the  orifices  of  the  ureters  above  and  of  the  ejaculatory 
ducts  below,  and  it  corresponds  therefore  with  what  is  termed  the 
trigotmm  vesica  together  with  the  floor  of  the  urethra  as  far  as  the 
openings  of  the  ejaculatory  ducts.  Throughout  this  region  the 
mucous  membrane  is  of  mesodermal  origin. 

The  urogenital  sinus  is  in  the  early  stages  also  tubular  in  its 
upper  part,  though  it  expands  considerably  below,  where  it  is 
closed  by  the  doacal  membrane.  This,  by  the  separation  of  the 
cloaca  into  rectum  and  sinus,  has  become  divided  into  two  portions, 
the  more  ventral  of  which  closes  the  sinus  and  the  dorsal  the  rectum, 
the  interval  between  them  having  become  considerably  thickened 
to  form  the  perineal  body.  In  embryos  of  about  17  mm.  the  uro- 
genital portion  of  the  membrane  has  broken  through,  and  in  later 
stages  the  tubular  portion  of  the  sinus  is  gradually  taken  up  Into 
the  more  expanded  lower  portion,  until  finally  the  entire  sinus  forms 
a  shallow  depression,  termed  the  vestibule,  into  the  upper  part  of 
which  the  urethra  opens,  while  below  are  the  openings  of  the 
Wolffian  (ejaculatory)  ducts  in  the  male  or  the  orifice  of  the  vagina 
in  the  female.  From  the  sides  of  the  lower  part  of  the  sinus  a  pair 
of  evaginations  arise  toward  the  end  of  the  fourth  month  and  give 
rise  to  the  bulbo-vestibular  glands  (Bartholin's)  of  the  female  or  the 
corresponding  bulbo-vretkral  glands  (Cowper's)  in  the  male. 


THE    EXTERNAL   GENITALIA  ,5, 

fifth^lf  Tl°^?*  "'  *^'  ^^'"^  W*nitaUa.-At  about  the 
«th  week  before  the  urogenital  sinus  has  opened  to  the  exterior 
he  mesenchyme  on  its  ventral  wall  begins  to  thicken,  produ  nga 
shght  projection  to  the  exterior.    This  eminence,  wWch  is  kZn 

extremity  b^omes  somewhat  bulbously  enlaced  (Fig  .  "  T^d 

ZTi:':^^^f'  '°j'^  ""^  "'  '"^  '--n^enlargeme",'  appears 

marked  gmtUU  folds  (Fig.  aai,  g/).    At  about  the  tenth  week  there 

S"  °°/*''  ^''''  •»'  ""^  *"•>«<='«  -°  enlargement  turned  the 
ge,^«velh„g  (Fig.  ,31,  ^.),  which  is  due  to  f  thickeSof  th! 
mesenchyme  of  the  lower  part  of  the  ventral  abdomina  3  n  tL 


f'°-  "f-Tai  EXTER.NAI  Genitalu  of  an  Embsvo  of  ,c  mw 

In  the  female  the  growth  of  the  genital  tubercle  proceeds  rather 
slowly  and  it  becomes  transformed  into  the  cliu,Hs,  thr^enhal  1 

it~EeT'°'°™l'''''*^'"''^'''-  ThegeniLswiS 
IfTnt^  J  lu  "'•^"^hyn'e  becomes  transformed  into  a  mass 
of  adipose  and  fibrous  tissue  and  they  become  converted  into  the 

uT'^"'.  "I'  "'^'"'"  '""''"="  *'"''■"  constituting  th" 
In  the  male  the  early  stages  of  development  are  closfly  siXTo 


364  TEOS   EXTXmNAL   GENITALIA 

those  of  the  female;  indeed,  it  has  been  well  said  that  the  external 
genitals  of  the  adult  female  resemble  those  of  the  fetal  male.  In 
early  stages  the  genital  tubercle  elongates  to  form  the  penis  and  the 
integument  which  covers  the  proximal  part  of  it  grows  forward  as  a 
fold  which  encloses  the  bulbous  enlargement  or  glans  and  forms  the 
prepuce,  whose  epithelium  fuses  with  that  covering  the  glans  and 
only  separates  from  it  later  by  a  comification  of  the  cells  along  the 
plane  of  fusion.  The  genital  folds  meet  together  and  fuse,  converting 
the  vestibule  and  the  groove  upon  the  vestibular  surface  of  the  penis 
into  the  terminal  portion  of  the  male  urethra  and  bringing  it  about 
that  the  vasa  deferentia  and  the  uterus  masculinus  open  upon  the 
floor  of  that  passage.  The  two  genital  swellings  are  at  the  same 
time  brought  closer  together,  so  as  to  lie  between  the  base  of  the 
penis  and  the  perineal  body  aAd,  eventually,  they  form  the  scrotum. 
The  mesenchyme  of  which  they  were  primarily  composed  differenti- 
ates into  the  same  layers  as  are  found  in  the  wall  of  the  abdomen  and 
a  peritoneal  pouch  is  prolonged  into  them  from  the  abdomen,  so  that 
they  form  sacs  into  which  the  testes  descend  toward  the  close  of  fetal 
life  (p.  366). 

The  homologies  of  the  portions  of  the  reproductive  apparatus 
derived  from  the  cloaca  and  of  the  external  genitalia  in  the  two  sexes 
may  be  perceived  from  the  following  table.  . 


Male 

Female 

Urinary  bladder. 

Urinary  bladder. 

Proximal  portion  of  urethra. 

Urethra. 

Bulbo-urethtal  glands. 

Bulbo-vestibular  glands. 

Urogenital  sinus 

The  rest  of  the  urethra. 

VesUbule. 

Genital  tubercle 

Penis. 

CUloris. 

Genital  folds 

Prepuce  and  integument  of 

perns. 

Labia  minora 

Genital  swellings.. . . 

Scrotum. 

Labia  majora. 

It  is  stated  above  that  the  layers  which  compose  the  walls  of  the  scro 
turn  are  identical  with  those  of  the  abdominal  wall.  This  may  be  seen  in 
detail  from  the  following  scheme: 


THE  DESCENT  OF  THE   OVARIES 


Abdohinal  WaIls. 
Int^utnent 
Superficial  fascia. 
External  oblique  muscle. 
Internal  oblique  muscle. 
Transverse  muscle. 
Peritoneum. 


36s 


SOIOTUH. 

Integument. 
Dartos. 

Intercolumnar  fascia. 
Cremasteric  fascia. 
Infundibuliform  fascia. 
Tunica  vaginalis. 


mcpmplete,  constituting  a  condition  known  as  AylSaeoSon 
which  offers  a  serious  bar  to  the  fulfilment  of  ihe  fexu^f'^t      If  ^h^ 

which  IS  usually  known  as  hermaphroditism.     It  is  noteworttivThaMn 

uterus  mascuhnus,  and  a  similar  condition  may  be  produced  i?  i 
female  by  an  excessive  development  of  the  clitoris.'^  Such  °a  ^howe  er 
which  concern  only  the  accessory  organs  of  reproduction.  are?ns7?nTes  of 

r^^rilT"  ""P*'*''  '"u""='^  '^'~*  l'ermiphroditism,l^X^^ut 
rochtism  bemg  a  terTi  which  should  be  reseVved  for  p.Se  ca^  in 
which  the  gemtal  ridges  give  rise  in  the  same  individual  1.^  ovf  L3 
spermatozoa.     Such  cases  are  of  eiceedine  rar"-         .h.    ..  * 

although  occasionaUy  observed  in  '^^:^:".  ,  t '  a"n7  thr/iill 
oTrs^ult  "^t  °'  hermaphroditisra  hit.,  to  ob^^-ell  t  S 

The  Descent  of  the  Ovaries  and  Testes.-The  positions 
finally  occupied  by  the  ovaries  and  testes  are  veiy  different  from 
those  which  they  possess  in  the  earlier  stages  of  development,  and 
this  IS  especially  true  in  the  case  of  the  testes.  The  change  of  position 
IS  partly  due  to  the  rate  of  growth  of  the  inguinal  ligaments  being 
ess  than  that  of  the  abdominal  walls,  the  reproductive  organs  being 
hereby  drawn  downward  toward  the  inguinal  regions  where  the 
hgaments  are  attached.  The  point  of  attachment  is  beneath  the 
bottorn  of  a  shght  pouch  of  peritoneum  which  projects  a  short  dis- 
ance  into  the  substance  of  the  genital  swellings  and  is  known  as  the 
can<U  a/Nuck  in  the  female,  and  in  the  male  as  the  vaginal  process 
In  the  female  a  second  factor  combines  with  that  just  mentioned 


366 


THE   DESCENT   OF   THE   TESTES 


The  relative  shortening  of  the  inguinal  ligaments  acting  alone 
would  draw  the  ovaries  toward'the  Inguinal  regions,  but  since  they 
are  united  to  the  uterus  by  the  ovarian  ligaments  movement  in  that 
direction  is  prevented  and  the  ovaries  come  to  lie  in  the  recto-uterine 
compartment  of  the  pelvic  cavity. 

With  the  testes  the  case  is  more  complicated,  since  in  addition  to 
the  relative  shortening  of  the  inguinal  ligaments  there  is  an  elonga- 
tion of  the  vaginal  processes  into  the  substance  of  the  genital  swell- 
ings, and  it  must  be  remembered  that  the  testes,  like  the  ovaries,  are 
primarily  connected  with  the  peritoneum.  Three  stages  may  be 
rerognized  in  the  descent  of  the  testes.    The  first  of  these  depends 


Fio.  aaa. — ^DiAORAKS  Iixustbating  the  Descent  or  the  Testis. 
U,  Inguinal  ligament;  m,  muscular  layer;  f,  skin  and  dartos  of  the  scrotum;  f,  testis; 
/v,  tumca  vaginalis;  W,  vas  deferens;  v^,  vaginal  processof  peritoneum. — (After  Htrtmg.) 

on  the  slow  rate  of  elongation  of  the  inguinal  ligaments  or  guber- 
nacula.  It  lasts  until  about  the  fifth  month  of  development,  when 
the  testes  lie  in  the  inguinal  repon  of  the  abdomen,  but  during  this 
month  the  elongation  of  the  gubemaculum  becomes  more  rapid  and 
brings  about  the  second  stage,  during  which  there  is  a  slight  ascent 
of  the  testes,  so  that  they  come  to  lie  a  little  higher  in  the  abdomen. 
This  stage  is,  however,  of  short  duration,  and  is  succeeded  by  the 
stage  of  the  final  descent,  which  is  characterized  by  the  elongation 
of  the  vaginal  processes  of  the  peritoneum  into  the  substance  of  the 
scrotum  (Fig.  222,  A).    Since  the  gubemaculum  is  attached  to  the 


THE   DESCENT   Ot  THE   TESTES 


367 


abdominal  wall  beneath  this  process,  and  since  its  growth  has  again 
diminished,  the  testes  gradually  assume  again  their  inguinal  posiUon, 
and  are  finally  drawn  down  into  the  scrotum  with  the  vuginal 
processes. 

The  condition  which  is  thus  acquired  persists  for  some  time  after 
birth,  the  testicles  being  readily  pushed  upward  into  the  abdominal 
cavity  along  the  cavity  by  which  they  descended.    Later,  however, 
the  size  of  the  openings  of  the  vaginal  processes  into  the  general 
pentoneal  cavity  becomes  greatly  reduced,  so  that  each  process 
becomes  converted  into  an  upper  narrow  neck  and  a  lower  sac-like 
cavity  (Fig.  222,  B),  and,  still  later,  the  walls  of  the  neck  portion  fuse 
and  become  converted  into  a  solid  cord,  while  the  lower  portion, 
wrapping  itself  around  the  testis,  becomes  the  tunica  vaginalis  (tv). 
By  these  changes  the  testes  become  permanently  located  in  the  scro- 
tum.   During  the  descent  of  the  testes  the  remains  of  each  Wolffian 
body,  the  epididymis,  and  the  upper  part  of  each  vas  deferens 
together  with  the  spermatic  vessels  and  nerves,  are  drawn  down  into 
the  scrotum,  and  the  mesenterial  fold  in  which  they  were  originally 
contained  also  practically  disappears,  becoming  converted  into  a 
sheath  of  connective  tissue  wliich  encloses  the  vas  deferens  and  the 
vessels  and  nerves,  binding  them  together  into  what  is  termed  the 
spermatic  cord.    The  mesorchium,  which  united  the  testis  to  the 
peritoneum  enclosing  the  Wolffian  body,  does  not  share  in  the  degen- 
eration of  the  latter,  but  persists  as  a  fold  extending  between  the 
epididymis  and  the  testis  and  forming  the  sinus  epididymis. 

In  the  text-books  of  anatomy  the  spermadc  cord  is  usuaUy  described 
as  lying  m  an  inguinal  canal  which  traverses  the  abdominal  walls  obliquely 
immedia  tely  above  Poupart's  Ugament.  So  long  as  the  lumen  ot  the  neck 
portion  of  the  vaginal  process  of  peritoneum  remains  patent  there  is  such 
a  canal,  placmg  the  cavity  of  the  tunica  vaginalis  in  communicadon  with 
the  general  pentoneal  cavity,  but  the  cord  does  not  traverse  this  canal, 
but  lies  outside  it  in  the  retroperitoneal  connective  tissue.  When 
however  the  neck  of  the  vaginal  proces..  disappears,  a  canal  no  longed 
exists,  although  the  connective  tissue  which  surrounds  the  spermadc 
cord  and  umtes  it  with  the  tissues  of  the  abdominal  walls  is  less  dense  than 
me  neighboring  tissues,  so  that  the  cord  may  readily  be  separated  from 
these  and  thus  appear  to  lie  in  a  canaL 


368 


IXTESATCU 


LITERATURE. 


B.  M.  Aluo):  "The  Embiyonic  Devdopmoil  of  the  Ovar/  and  Te»te»  in  Munmalt," 

Amu.  Jam.  0/ Anal.,  m,  1904. 
J.  L.  BiEim:  "Morphology  of  the  Tubulct  of  the  Human  Te»ti»  and  Epididjmii,' 

Amer.  Jam.  Anat.,  XI,  1911. 
F.  J.  EvATi:  "A  Contribution  to  the  Developmait  of  the  Prottate  in  Man,"  Jam. 

Anal,  md  Phys.,  xua,  1909. 
E.  J.  EvAii:  "  A  Contribution  to  th?  Development  of  the  Proiute  Gland  in  the  Human 

Female,"  Joarn.  Atua.  and  Phys.,  XLV,  1911. 
W.  FlUX:  "  Entwididungsgeschichte  del  Exkretions-iystems,"  Erflm.  dtr  Ama.  md 

EntwkUungsgack.,  xin,  1903. 
W.  FiLix:  "Die  Entwicklung  del  Ham-  und  Geachlechtiorgane,"  in  KxiBIl-llAU. 

Human  Embryology,  II,  1911. 
■  A.  FLliaciniANN:  "Morphologische  Studieu  ttber  Kloake  und  PhaUus  der  Amnioten, 

ilortkol.  Jarklmck,  xxx,  xxxil  und  xxxvi,  190J,  1904,  1907. 
O.  FaANKl:  "  Beitiilge  jur  Lehre  vom  Descensus  traticukmun,"  SUmntsbtr.  dtr  kail. 

Akad.  WUsmsck.  Wim,  Malk.-Nalifimss.  Claat,  cix,  1900. 
S.  P.  Gaoe:  "A  Three  Weeks  Human  Embryo,  with  especial  reference  to  the  Brain 

and  the  Nephric  System,"  Amer.  Joum.  of  Anat.,  iv,  1905. 
D.  B.  Habt:  "The  Nature  and  Cause  of  the  Physiological  Descent  of  the  Testes," 

Jaum.  Anat.  and  Phys.,  xuv,  1909. 

D.  B.  Haxt:  "  The  Physiolopcal  Descent  of  the  Ovaries  in  the  Human  Foetus,"    Jonm. 

Anal,  and  Phys.,  xuv,  1909. 

E.  Hauch:  "Ueber  die  Anatomie  und  EntwicUung  der  Nieren,"  Anal.  Htjtt,  xxn, 

1903. 
G.  C.  HireM:  "On  the  Development  and  Shape  of  the  Uriniferous  Tubules  of  Certain 

of  the  Higher  Mammals,"  Ama.  Joum.  of  Anat.,  IV,  Suppl.  1905. 
J.  Janosik:  "Histolopsch-embryologjsche  Untersuchungen  ttber  das  Uiogenitalsystem," 

SilMungsher.  der  kais.  Akad.  WUsensch.  Wien,  Ualk.-Naturwiss.  CfcuM,  xci,  1887 
J.  Janosik:  "Ueber  die  Entwicklung  der  Nachniere  bd  den  Amnioten,"  Arch,  fir 

Anal.  u.  Phys.,  Anal.  Ablk.,  1907. 
J.  JAN03IK:  "Entwicklung  de»  Nierenbeckens  beim  Maischen,"  Arch.  f»  mihrosk. 

Anal.,  Lxxviu,  1911.  __ 

F.  KzmzL:  "Zur  Entwickelungsgeschichte  des  menschlichen  Urogenital-apparatua, 

Archivftir  Anal,  md  PhysM.,  Anat.  Ablh.,  1896. 
J.  B.  Macalluh:  "  Notes  on  the  WolfSan  Body  of  Higher  Mammals,"  Amer.  Joum.  o 

Anal.,  I,  1903. 
E.  Martin:  "Ueber  die  Anlage  der  Umiere  beim  Kaninchen,"  ArcMv  /tr  Anal,  und 

Physiol.,  Anal.  AM.,  1888. 
H.  Mxyxr:  "  Die  Entwickelung  der  Umieren  beim  Menschen,"  ArcUv  /Br  mikrosk. 

Anat.,  xxxvi,  1890. 
R.  Miyi..:  "Zur  Kenntnis  des  Gartner'schen  Ganges  besondera  in  der  Vagina  und 

dem  Hymen  des  Menschen,"  Arch.filr  mikrosk.  Anal.,  Lxxm,  1909. 
R.  Hxter:  "Zur  Entwicklungsgeachichte  und  Anatomie  des  utriculus  prosUticus  beim 

Menschen,"  Arch,  fir  mikrosk.  Anal,  vaar,  1909 


inuATosx 


369 


»4 


f 


CHAPTER  XIV. 
THE  SUPRAREHAL  SYSTEM  OF  ORGANS. 

To  the  suprarenal  system  a  number  of  bodies  of  peculiar  struc- 
ture, probably  concerned  with  internal  secretion,  may  be  assigned. 
In  the  fishes  they  fall  into  two  distinct  groups,  the  one  containing 
organs  derived  from  the  coelomic  epithelium  and  known  as  interrenal 
organs,  and  the  other  consisting  of  organs  derived  from  the  sym- 
pathetic nervous  system  and  which,  on  account  of  the  characteristic 
affinity  they  possess  for  chromium  salts,  have  been  termed  the 
chromaffine  organs.  But  in  the  amphibia  and  amniote  vertebrates, 
while  both  the  groups  are  represented  by  independent  organs,  yet 
they  also  become  intimately  associated  to  form  the  mfraretuU  bodies, 
so  that,  notwithstanding  their  distinctly  different  origins,  it  is 
convenient  to  consider  them  together. 

'^  .  Development  of  the  Suprarenal  Bodies.— The  supra- 
renal bodies  make  their  appearance  at  an  early  stage,  while  the 
Wolffian  bodies  are  still  in  a  well-developed  condition,  and  they  are 
situated  at  first  to  the  medial  ade  of  the  upper  ends  of  these  struc 
tures  (Fig.  an,  ir).  Their  final  relation  to  the  metanephros  is  a 
secondary  event,  and  is  merely  a  topographic  relation,  there  being 
no  developmental  relation  between  the  two  structures. 

In  the  human  embryo  they  make  their  appearance  at  about  the 
beginning  of  the  fourth  week  of  development  as  a  number  of  pro- 
liferations of  the  coelomic  epithelium,  which  project  into  the  sub- 
jacent mesenchyme,  and  are  situated  on  either  side  of  the  median 
line  between  the  root  of  the  mesentery  and  the  upper  portion  of  the 
Wolffian  body.  The  various  proliferations  soon  separate  from  the 
epithelium  and  unite  to  form  two  masses  situated  in  the  mesenchyme, 
one  on  either  side  of  the  upper  portion  of  the  abdominal  aorta.  In 
certain  forms,  such  as  the  rabbit,  the  primary  proliferations  arise 
370 


DBVEIOPMENT   O,  THE   Srw*„N«  BODIES  37, 

S„'l^h?°"°'*'?"^'°"'°'''''^«'°'»''=  ^''''helium  (Fig  „,) 
but  in  the  humaa  embryo  these  depressions  C.  ■  ,t  form  ^^' 

strands  traveisine  them     At  <,k„,  .  .u      .    ^'^"e-  *24;>  and  form 

•ntermingled  for  a  considerable  t,-me.  a„S  r„r;;r:^^^^^^^^^^ 


'     ""^  "■  "'^■^oK^.r^rTuB^^--^-™',  vein;  ».  .B„,.  oT 

etl*';UuXt:c'::trS^       ^'«'  ^^-p-^^^c 

imbedded   k   the   mTH,,  ."'   ""'"■'""'    **^^"«   '«">^ "»« 

some  tracing  ttidr  oriSn  l^u"''  "  '"  f"'  °'  t^e  excretory  apparatus 


37a 


oivxu>piaNT  or  the  iupiaixhai.  bodiu 


formed  from  the  bottom  of  deprenioiu  of  the  coelomic  epithelium  lecmed 
to  lend  iupport  to  this  view,  but  it  i»  now  pretty  firmly  established  that 
the  appearances  thus  presented  do  not  warrant  the  interpietation  placed 
upon  them  and  that  the  interrenal  tissue  is  derived  from  the  ccelomic 
epithelium  quite  independently  of  the  nephric  tubules.  That  the  chrom- 
aSine  tissue  is  a  derivative  of  the  sympathetic  nervous  system  has  long 
been  recognized. 

During  the  development  of  the  suprarenal  glands  portions  of 
their  tissue  may  be  separated  as  the  result  of  unequal  growth  and 
form  what  are  commonly  spoken  of  as  accessory  suprarenal  glands, 
although,  since  they  are  usually  composed  solely  of  cortical  sub- 


Fio.  JJ4.— SicnoN  THionoH  the  Snp«A»iHAL  Body  o»  ah  Ehbixo  o>  17  iu. 

A,  Aoru;  R,  lntciren«l  portion;  S,  iTmpttbetic  nervous  syitcm;5B,  ■jrmpathetk  celli 

penetrating  the  mtenenal  portion. — O^Usti,) 

Stance,  the  term  accessory  interrenal  bodies  would  be  more  appropriati . 
They  may  be  formed  at  different  periods  of  development  and  occur 
in  various  situations,  as  for  instance,  in  the  vicinity  of  the  kidne\s 
or  even  actually  imbedded  in  their  substance,  on  the  walls  of  neigh- 
boring blood-vessels,  in  the  retroperitoneal  tissue  below  the  level ..[ 
the  kidneys,  and  in  connectici  with  the  organs  of  reproduction,  in 
the  spermatic  cord,  epididymi.  or  rete  testis  of  the  male  and  in  tie 
broad  ligament  of  the  female. 

It  seems  probable  that  the  bodies  associated  with  the  reproducti  -e 


MV.lOW.Ht  OF  1H.  .OTlAKKAt  BODIM  3., 

apparatus  are  sepwated  from  the  main  m««  of  In.^    1  ., 
before  the  immigration  of  the  ^ymZLTL^L^TZf  'T 
descent  of  the  ovari«  nr  »«..-    J'™l««n«>c  tissue  and  before  the 

level,  are  o  literal    'Tr*        '  """^  "^'^  '^^^  «'  higher 

bodTs  are.  how:;erX  ^^ iv^;  rl^eTe'lIT ™"'''-    '"=" 

independent  chromaffine  o.:gans  also  occur,  among  them  the 


cHromalecrhavtrili^^^   2i"-^^^  ^^^  "?' 
7^  /«/^ca..tf<f  G<«,;i«,._These  structures,  which  are  fre- 


'i 


374 


THE   DiTEKCAaOTID   QANGLIA 


quently  though  incorrectly  tenned  carotid  glands,  are  small  bodies 
about  5  mm.  in  length,  which  lie  usually  to  the  meaijt  side  of  the 
upper  ends  of  the  common  carotid  arteries.  They  ^ft«ess  a  very 
rich  arterial  supply  and  stand  in  intimate  relation  with  the  branches 
of  an  intercarotid  sympathetic  plexus,  and,  furthermore,  they  are 
characterized  by  possessing  as  their  specific  constituents  markedly 
chromaffine  cells,  among  which  are  scattered  stellate  cells  resembling 
the  cells  of  the  sympathetic  ganglia. 

They  have  been  found  to  arise  in  pig  embryos  of  44  mm.  by  the 
separation  of  cells  from  the  ganglionic  masses  scattered  throughout 
the  carotid  sympathetic  plexuses.  These  cells,  which  become  the 
chromaflSne  cells,  arrange  themselves  in  round  masses  termed  cell 
balls,  many  of  which  unite  to  form  each  ganglion,  and  in  man  each 
cell  ball  becomes  broken  up  into  trabecula  by  the  blood-vessels 
(Fig.  225)  which  penetrate  its  substance,  and  the  individual  balls  are 
separated  from  one  another  by  considerable  quantities  of  connective 
tissue. 

Some  confusion  has  existed  in  the  past  as  to  the  origin  of  this  structure. 
The  mesial  wall  of  the  proximal  part  of  the  internal  carotid  artery  becomes 
considerably  thickened  during  the  early  stages  of  development  and  the 
thickening  is  traversed  by  numerous  blood  lacume  which  communicate 
with  the  lumen  of  the  vessel.  This  condition  is  perhaps  a  relic  of  the 
branchial  capillaries  which  in  the  lower  gill-breathing  vertebrates  repre- 
sent the  proximal  portion  of  the  internal  carotid,  and  has  nothing  to  do 
with  the  formation  of  the  intercarotid  ganglion,  although  it  has  been 
believed  by  some  authors  (Schaper)  that  the  ganglion  was  derived  from 
the  thickening  of  the  wall  of  the  vessel.  The  fact  that  in  some  animals, 
such  as  the  rat  and  the  dog,  die  ganglion  stands  in  relation  with  the 
external  carotid  and  receives  its  blood-supply  from  that  vessel  is  of  im- 
portance in  this  connection. 

The  thickening  of  the  internal  carotid  disappears  in  the  higher 
vertebrates  almost  entirely,  but  in  the  Amphibia  it  persists  throughout 
life,  the  lumen  of  the  proximal  part  of  the  vessel  being  converted  into  a 
fine  ms^work  by  the  numerous  trabecule  which  traverse  it.  This 
carotid  labyrinth  has  been  termed  the  carotid  gland,  a  circumstance 
which  has  probably  assisted  in  producing  confusion  as  to  the  real  signifi- 
cance of  the  intercarotid  ganglion. 

The  Organs  of  Zuckerkandl.—ln  embryos  of  14.5  mm.  there 
have  been  fwind,  in  front  of  the  abdominal  aorta,  closely  packed 


THE    OBCANS   OF   ZUCKERKANDL  375 

ZTJ-  ''■''7'^'^  '*"■"""  '»  "PP^'^'^'^  'he  cells  composing 

along  he  side  of  the  aorU  to  below  the  point  of  origin  of  thelnfrrior 
mesentenc  artery,  being  especially  distinct.  Thesf  cell  ZuTZ 
nse  to  the  ganglia  of  the  p«vertebral  sympathetic  plexus^aTd  flo 


<•  Aorta" Vt'"^'"'""  °'  ZvcK^ju^oL  «oH  A  New-bo^,  Ceap. 
left  renal  nia.-iZuckettamii:)  ^"^^  ™«™'  #'".  """ic  plexie;  «,  ureter;  t,.r.j, 

'0  peculiar  bodies  which,  from  their  discoverer  mav  be  trrn^H  ... 

SIS  !^tt^"'"^H"^^^•-^^  ^'--^ '"  ^"'^^^^^^^^^^^ 

resemuL     Jh  'y"P^">«'«=  P'*™^*'  »d  has  a  rich  blood-supply, 
resembhng  w  these_respe<-K  the  intercarotid  ganglia,  and  the  resem 


if  11 


376 


imSATDKZ 


blance  is  further  increased  by  the  fact  that  the  specific  cells  of  the 
organ  are  markedly  chromaffine. 

«i  At  birth  the  bodies  situated  m  the  upper  portion  of  the  abdominal 
cavity  have  broken  up  into  small  masses,  but  the  two  lower  ones, 
mentioned  above,  are  stUl  well  defined  (Fig.  326).  Even  these,  how- 
ever, seem  to  disappear  later  on  and  no  traces  of  them  have  as  yet 
been  found  in  the  adult. 


UTERATURE. 

A.  Kohn:  "Ueber  da  B»u  lind  die  Entwickdimg  der  sog.  Carotisdiase,"  Arckiv. 
filr  mikrosk.  Atul.,  LVl,  190a  ,  „      .  u.  i 

A.  Kohn:  "Das  chiomaffine  Gewebe,"  &«<*».  <««■  Anat.  «nd  EntwiM»,t!gesck.. 

H    p"lL-^Die  vergleicha.de  Entwi^ung.ge«iichte  der  NebemuereMyrteme  der 
■    Wirbeldere,"  Hertrnt's  Handb.  *r  vtrgl.  md  exper.  EMwkkbmgslekre  der  W.rbel- 

titrt,  m,  1906.  ,    ,       1.      1 

A    Souui:   "Recherclies  sur  le  dtveloppement  des  capsules  surrtoales  chei  les 

WoMbrfs,"  Joum.de  tAnat.et  ilia  PkysM.,Taaax,i<)Ol-  „ 

J.  Whsel:  "Beitr»ge  air  Anatomie  and  Entwidteluiig  der  menschlichen  Nebenniere, 

Anat.  Heft.,  MX,  190a.  _  .         , 

E    ZoCEnulANDL:    "Ueber  Nebenoigane  des  Sympathicu.  im  Retroperitonealiaum 

des  Menschen,"  Verkandl.  Anat.  GeseUsck.,  XV,  1901. 


CHAPTER  XV. 

THE  DEVELOPKENT  OF  THE  ITERVOUS 
SYSTEM. 

The  Histogenesis  of  the  Nervous  System.— The  entire  central 
nervous  system  is  derived  from  the  celk  h'ning  the  medullary  groove, 
whose  formation  and  conversion  into  the  medullary  canal  has  abeady 
been  described  (p.  72).    When  the  groove  is  first  formed,  the  cells 
lining  it  are  somewhat  more  columnar  in  shape  than  those  on  either 
side  of  it,  though  like  them  they  are  arranged  in  a  single  layer; 
later  they  increase  by  mitotic  division  and  arrange  themselves  in 
several  layers,  so  that  the  ectoderm  of  the  groove  becomes  very  much 
thicker  than  that  of  the  general  surface  of  the  body.    At  the  same 
time  the  cell  boundaries,  which  were  originally  quite  distinct, 
gradually  disappear,  the  tissue  becoming  a  syncytium.    While  its 
tissue  is  in  this  condition  the  lips  of  the  medullary  groove  unite, 
and  the  subsequent  differentiation  of  the  canal  so  formed  differs 
somewhat  in  different  regions,  although  a  fundamental  plan  may  be 
recognized.    This  plan  is  most  readily  perceived  in  the  region  which 
becomes  the  spinal  cord,  and  may  be  described  as  seen  in  that  region. 
Throughout  the  eariier  stages,  the  cells  lining  the  inner  wall  of 
the  medullary  tube  are  found  in  active  proliferation,  some  of  the 
cells  so  produced  arranging  themselves  with  their  long  axes  at  right 
angles  to  the  central  canal  (Fig.  227),  while  others,  whose  destiny 
is  for  the  most  part  not  yet  determinable,  and  which  therefore  may 
be  termed  indifferent  ceUs  are  scattered  throughout  the  syncytium. 
At  this  stage  a  transverse  section  of  the  medullary  tube  shows  it  to 
be  composed  of  two  well-defined  zones,  an  inner  one  immediately 
surrounding  the  central  canal  and  composed  of  the  indifferent  cells 
and  the  bodies  of  the  inner  or  ependymal  cells,  and  an  outer  one  con- 
sisting of  branched  prolongations  of  the  syncytial  cytoplasm.    This 
377 


378 


THE   HISTOGENESIS   Of  THE   NERVOUS   SYSTEM 


outer  layer  is  termed  the  margmal  velum  (RandscMeier)  (Fig.  337, 
m).  The  indifferent  cells  now  begin  to  wander  outward  to  form 
a  definite  layer,  termed  the  manlU  layer,  lying  between  the  marginal 
velum  and  the  bodies  of  the  ependymal  cells  (Fig.  21S),  and  when 
this  layer  has  become  well  established  the  cells  composing  it  begin 
to  divide  and  to  differentiate  into  (i)  cells  termed  neuroblasts, 
destined  to  become  nerve-cells,  and  (a)  others  which  appear  to  be 
supportive  in  character  and  are  termed  neuroglia  cells  (Fig.  228,  B). 


I  It' 


1 


m 


Flo.  337.— Tkans VERSE  Section  thkough  the  Spinal  Cokd  of  a  Pig  Eubkyo 
or  30  int.,  THE  Uppek  Fakt  showing  the  Appearance  produced  by  the  Silver 
Method  op  Dehonstrating  the  Neuroglia  Fibers. 

a,  Epcndyma  of  floor  plate;  b,  boundary  between  mantle  layer  and  marginal 
zone;  cs,  mfflenchyraal  connective-tissue  synt^tium;  ep,  ependymal  cells;  i,  ingrowth 
of  connective  tissue;  m,  marginal  velum;  mn,  mantle  layer;  mv,  mantle  layer  ol  floor 
I^te;  p,  pia  mater;  r,  neuroglia  fibers.— -(Korii«/y.) 

The  latter  are  for  the  most  part  small  and  are  scattered  among  the 
neuroblasts,  these,  on  the  other  hand,  being  laiger  and  each  early 
developing  a  single  strong  process  which  grows  out  into  the  marginal 
velum  and  is  known  as  an  aods-cyUnder,    At  a  later  period  the 


THE  mSTOOENESlS  OF  THE  NERVOUS   SYSTEM  379 

neuroblasts  abo  give  rise  to  other  processes,  termed  dendrUes,  more 
slender  and  shorter  than  the  aris-cyhnders.  branching  repeatedly 
and,  as  a  rule,  not  extending  beyond  the  limits  of  the  mantle  layer 
In  connection  with  the  neuroglia  cells  peculiar  neuroglia  fibrils 
develop  very  much  in  the  same  way  as  the  fibers  are  formed  in  mesen- 
chymal connective  tissue.  That  is  to  say,  they  are  formed  from  the 
peripheral  portions  of  the  cytoplasm  of  the  neuroglial  and  ependy- 
mal  cells.  But  since  these  cells  are  connected  together  to  form  a 
syncytium  the  tibrils  are  not  confined  to  the  territories  of  the  indi- 


Fk..  »8.-Du<!«ahs  sHowmo  tb«  Deveiophknt  of  ihz  Mantix  Laym  m  ra. 
Spinal  Coed.  ^^ 

vidual  cells,  but  may  extend  far  beyond  these,  passing  in  the  syncv- 
mm  from  the  territory  of  one  neuroglial  cell  to  another,  many  of 
those,  mdeed,  arising  in  connection  with  the  ependymal  cells  extend- 
ing throughout  the  entire  thickness  of  the  medullary  wall  (Fig  227) 
The  fibrils  branch  abundantly  and  form  a  supportive  network 
extending  through  all  portions  of  the  central  nervous  system. 
The  axis-cylinder  processes  of  the  majority  of  the  neuroblasts  on 
reaching  the  marginal  velum  bend  upward  or  downward  and,  after 


li 


38o 


THE  KI8T0OBNE8IS  OF  THE  MUTODS  SYSTEM 


traversing  a  greater  or  less  length  of  the  cord,  re-enter  the  mantle 
layer  and  terminate  by  dividing  into  numerous  short  branches  which 
come  into  relation  with  the  dendrites  of  adjacent  neuroblasts. 
The  processes  of  certain  cells  situated  in  the  ventral  region  of  the 
mantle  zone  pass,  however,  directly  through  the  marginal  velum 
out  into  the  surrounding  tissues  and  constitute  the  ventral  nerve- 
roots  (Fig.  231). 

The  dorsal  nerve-roots  have  a  very  different  origin.  In  embryos 
of  about  3.5  mm.,  in  which  the 
medullary  canal  is  only  partly 
closed  (Fig.  53),  the  celb  which 
lie  along  the  line  of  transition 
between  the  lips  of  the  groove 
and  the  general  ectoderm  form 
a  distinct  ridge  readily  recog- 
nized in  sections  and  termed  the 
neural  crest  (Fig.  339,  A).  When 
the  lips  of  the  groove  fuse  to- 
gether the  cells  of  the  crest  unite 
to  form  a  wedge-shaped  mass, 
completing  the  closure  of  the 
canal  (Fig.  339,  B),  and  later 
proiifp'^te  so  as  to  extend  out- 
ward over  the  surface  of  the 
canal  (Fig.  339,  C).  Since  this 
proliferation  is  most  active  in  the 
regions  of  the  crest  which  corre- 
spond to  the  mesodermic  somites 
there  is  formed  a  series  of  cell  masses,  arranged  segmentally 
and  situated  in  the  mesenchyme  at  the  sides  of  the  medullary 
canal  (Fig.  314).  These  cell  masses  represent  the  dorsal  root 
ganglia,  and  certain  of  their  constituent  cells,  which  may  also  be 
termed  neuroblasts,  early  assume  a  fusiform  shape  and  send  out  a 
process  from  each  extremity.  One  of  these  processes,  the  axis 
cylinder,  grows  inward  toward  the  medullary  cana)  ant*  penetrates  its 


Fig.  aag. — Twrbe  Sections  through 
THE  MzDDUJUty  Canal  or  ah  Eiibryo 
OF  a.5  inf. — (vonLekhossek^ 


THE  HISTO0ENK8I8  OF  IHK  ia«V0IJ8  SYSTKlf 


381 


ma^nd  velum  and,  after  a  longer  or  shorter  course  in  this  rone, 
eaters  lie  mantle  layer  and  comes  into  contact  with  the  dendrites  o 
.Tin  '•'V»'«J  "^-wblasts.    The  other  process  extends  per- 

•S^Tlt  '?  '°  '^  ««'"^«^  "an  extremely  elongated  dendL 
The  processes  from  the  cells  of  each  ganglion  aggregate  to  form  a 
nem  Uut  formed  by  the  axis-cyUnders  being  the  posterior  root  of 
a  spinal  nerve,  while  that  formed  by  the  dendrites  soon  unites  with 

Lain  st™  r""':'?  °^  '^'  corresponding  segment  to  form  the 
mam  stem  of  a  spmal  nerve. 

^J^T.t  "■"'  "  """^  taportant  difference  in  the  mode  of  develop- 
ment of  the  two  nerve-roots,  the  axis-cylinders  of  the  ventral  1006 


/wiJ^I^  7°"  ™  ^*^'^  GANGL.OK  or  A  Gm«*.P.O  EHB.YO 

arising  from  cells  situated  in  the  wall  of  the  medullary  canal  and  erow- 
mg  outward  (centrifugaUy),  while  those  of  the  dorsal  root  spring 
rom  cells  situated  peripherally  and  grow  inward  (centripeteUy) 
toward  the  medullan^  canal.  In  the  majority  of  the  do^l  ro^i 
ganglia  the  pomts  of  origin  of  the  two  processes  of  each  bi-polar 
ceU  graduaUy  approach  one  another  (Fig.  230,  4)  and  eventually 
come  to  nse  from  a  common  stem,  a  process  of  the  cell-body,  which 
thus  assumes  a  characteristic  T  form  (Fig.  230,  d). 

an  fu'Z.I^f ''"  been  said  it  wiU  be  seen  that  each  axis-cylinder  is 
at,  K^H?,f  "rt  "'"5"'''^ '  ""^  ^  part  of  its  ceU-bodjTrarl 
also  the  dendrites.    Another  view  has,  however,  been  advanci  to  Se 


i  i 


-  J 

^      ; 

1 

H 

383  THE  BUTOOENZnS  01  THE  MEEVOUS   SVSTEU 

effect  that  the  nerve  fibers  first  appear  as  chains  of  cells  and  that  the  azif- 
cylinders,  being  differentiated  from  the  cytoplasm  of  the  chains,  are  really 
multicellular  products.  Many  difficulties  stand  in  the  way  of  the  ac- 
ceptance of  this  view  and  recent  observations,  both  histogenetic  (Cajal) 
and  experimental  (Harrison),  tend  to  confirm  the  unicellular  origin  of 
the  axis-cylindera.  The  embtyobgical  evidence  therefore  goes  to  support 
^e  neurone  theory,  which  regarcb  the  entire  nervous  system  as  com- 
posed of  definite  units,  each  of  which  corresponds  to  a  single  cell  and  is 
termed  a  neurone. 

By  the  development  of  the  axis-cylinders  which  occupy  the  meshes 
of  the  marginal  velum,  that  zone  increases  in  thickness  and  comes 
to  consist  principally  of  nerve-fibers,  while  the  cell-bodies  of  the 
neurones  of  the  cord  are  sittiat^  in  the  mantle  zone.  No  such  de- 
finite distinction  of  color  in  the  two  zones  as  exists  in  the  adult  is, 
however,  noticeable  until  a  late  period  of  development,  the  meduUary 
sheaths,  which  give  to  the  nerve-fibers  their  white  appearance  not 
beginning  to  appear  until  the  fifth  month  and  continuing  to  form 
from  that  time  onward  until  after  birth.  The  origin  of  the  myelin 
which  composes  the  medullary  sheaths  is  as  yet  uncertain,  although 
the  more  recent  observations  tend  to  show  that  it  is  picked  out  from 
the  blood  and  deposited  around  the  axis-cylinders  in  some  manner 
not  yet  understood.  Its  appearance  is  of  importance  as  being 
associated  with  the  beginning  of  the  functional  activity  of  the 
nerve-fibers. 

In  addition  to  the  medullary  sheaths  the  majority  of  the  fibers 
of  the  peripheral  nervous  system  are  provided  with  primitive  sheaths, 
which  are  lacking,  however,  to  the  fibers  of  the  central  sjrstem. 
They  are  f<  rmed  by  cells  which  wander  out  from  the  dorsal 
root-ganglia  and  are  therefore  of  ectodermal  origin.  Frog  larvs 
deprived  of  their  neural  crests  at  an  early  stage  of  development 
pr-  iuce  ventral  nerve-fibers  altogether  destitute  of  primitive 
sheaths  (Harrison). 

Various  theories  have  been  advanced  to  account  for  the  formation  of 
the  medullary  sheaths.  It  has  been  held  that  the  myelin  is  formed  at  thi 
expense  of  the  outermost  portions  of  the  axis-cylinders  themselves  (von 
KoUiker),  and  on  the  other  hand,  it  has  been  regarded  as  an  excretior 
of  the  cdls  which  compose  the  primitive  sheaths  surrounding  the  fibers 


IRE  SPINAL  OntD 


383 


primitive  dieath,.    As  ,uted  tCf  X  l''"'""  "''^'^P"""'  ««> 
(WlMsIc)  indicate  it.  L^„ou.oS.  ""      ob«rv.tio„. 

It  has  been  seen  that  the  central  canal  is  closed  in  the  n,id-dorsal 
Ime  by  a  nuiss  of  cells  derived  from  the  neural  crest.    These  cdls 

c^ol  t'l     ^^  "."'^  '°"^"°°  •"  ">^  '-'""^  ^y"-  ''U  "com 
thZn    rT'?"''^J°'°  'P""*^""' '"""'' ""«» 'he  same  is  true  of 
the  cells  situated  m  the  mid-vential  line  of  the  canal.    In  these  two 

^Z\SZLrJ''  r-^-^  and>..^„,.  respective;^'! 
wall  of  the  canal  has  a  characteristic  structure  and  does  not  share 
o  any  great  extent  in  the  increase  of  thickness  which  distinguish 
he  other  reg,o„s  (Fig.  ,3,).  i„  the  lateral  waUs  of  the  canaHhere 
.  also  nofceable  a  differentiation  into  two  regions,  a  dorsal  one 
sumdrng  m  relation  to  the  ingrowing  fibers  from  the  dot^  r" 
pmgha  and  known  as  the  dorsal  .one,  and  a  ventral  one,  the  vS^ 
zone,  similarly  related  to  the  ventral  nerve-roots,    in  diff^ent 

floor-pla tes,  undergo  different  degrees  of  development,  producing 
pecuhanties  which  may  now  be  considered 

of  t^  m^rf^'"*  °^*•  ^J""''  Cord-Even  before  the  lips 
of  the  medullary  groove  have  met  a  marked  enla^fement  of  the 

C^tZT-  t  *■=  ?'"' '^  "»«=-'"<>'  'he  regTon  which  will 
become  the  bram  bemg  thus  distinguished  ft»m  the  more  posterior 
portion  which  will  be  converted  into  the  spinal  cord.  Xn  "he 
formation  of  the  mesodermic  somites  is  completed,  the  spinal  corf 

^W  w  "T  ""'^  '^'  ^"^'=™  °^  'he  dorsal  surface  of 
the  body;  but  m  that  portion  of  the  cord  which  is  posterior  to  the 

Z.!rZ^  .'!*^'"'  """  histological  differentiation  does  not 
proceed  beyond  the  stage  when  the  walls  consist  of  several  layers  of 
similar  cells,  the  formation  of  neuroblasts  and  nerve-roots  celling 
«-ith  the  segment  named.  After  the  fourth  month  the  more  differ 
entiated  portion  elongates  at  a  much  slower  rate  than  the  surround- 
ing tissues  and  so  appears  to  recede  up  the  spinal  canal,  until  its 


t 


384 


THE   SFIMAL  COID 


termination  i.  opposite  the  second  lumber  vertebra^  The  l«. 
S::^.iatedpoXTwhichretainsitsconnectionw.thA^^^^^^ 

until  about  the  fifth  month,  is,  on  the  other  h^"<J' f  »*"^°"' '"'"  ' 
Inder  filament  whose  ceUs  degenerate  dunng  '»»«*"";  "»«»*• 
except  in  its  uppermost  part,  so  that  it  comes  to  be  represented 
Shout  the'^eater  part  of  iU  extent  by  a  *-  -rd  co™pK«ed 
of  Dia  mater  This  cord  is  the  structure  known  m  the  adult  as  he 
iuZ^U  and  lies  in  the  center  of  a  ^-h  of  nerves^cupymg 
the  lower  part  of  the  spinal  canal  and  termed  the  «"^  '««»?»; 
The  existence  of  the  cauda  is  due  to  the  recession  of  ^e  cordjh.ch 
necessitates  for  the  lower  lumbar,  sacral  and  coccygeal  nerves,  a 
Ts^  nt Trough  the  spinal  canal  for  a  greater  or  less  d« 
Se  they  can  reach  the  intervertebral  foramina  through  which 

*'''ln"Th:SSges  of  development  the  central  canal  of  the  cord 
is  i:  laSe  Ld  li  an  elongated  oval  'o-  J'-J^^'-;^^- 
somewhat  rhomboidal  in  shape  (F.g.  .3^,  A),  the  lateja  a^gtes 
marking  the  boundaries  between  the  dorsal  and  ventral  zones^ 
Mdevflopment  proceeds  the  sides  of  the  canal  in  the  dorsal  repon 
J^dSy  approach  one  another  and  eventuaUy  fuse  so  that  th» 
Trtten  of  the  canal  becomes  obliterated  (Fig  .3'.  B)  «d  ^^^ 
Sby  the  dorsal  longitudinal  fissure  in  the  adult  cord  the  cental 
anal  o  which  corresponds  to  the  ventral  portion  or^V^i^^^; 
onic  cavity.  While  this  process  has  been  gomg  on  bo  h  the  ««f 
and  the  flL-pUte  have  become  depressed  below  *e  -1  MJhe 
general  surface  of  the  cord,  and  by  a  contmuance  of  the  depression 
of  the  floor-plate-^  process  really  due  to  the  enlargement  and 

loX^^  bulging  o' *«  -'-'  "'^^^  nr^tle  d^l 
is  produced,  the  difierence  between  its  shape  and  that  of  the  dorsal 

fissure  being  due  to  the  difference  in  its  development. 

The  development  of  the  mantle  layer  P^^^-^^J*  J"^  ^°" 

rapidly  in  the  ventral  zone  than  in  the  dorsal,  so  that  at  an  ear^ 

Ze  ^ig.  .31,  A)  the  anterior  column  of  gray  matter  is  much  more 

pTonoun^d  bU  on  the  development  of  the  dorsal  ne,:ve-roots  the 

Eron  o  neuroblasts  in  the  dorsal  zone  proceeds  apace,  resulting 


THE   SPINAL  COSD  jg. 

in  the  formation  of  a  dorsal  column.  A  smoll  portion  of  the  zone, 
situated  between  the  point  of  entrance  of  the  dorsal  nerve-roots  and 
the  roof-plate,  fails,  however,  to  give  rise  to  neuroblasts  and  is 
entirely  converted  into  ependyma.  This  represents  the  future 
fymadus  gracilis  (fasciculus  of  CoU)  (Fig.  J31,  A.  cG),  aiic'  it  the 
pomt  of  entrance  of  the  dorsal  roots  into  the  cord  a  well-marked 
oval  bundle  of  fibers  is  formed  (Fig.  331,  A,  ob)  which,  as  develop- 


^"''  '^'TI.^^'^'^  Sections  ih«ouoh  the  Spdjai  Cords  of  Embryos  or  lA) 

ABOUT  Four  ANT,  A  Half  Weeks  AND  (B)  ABOHT  Three  Months 
z.m^VS^'""'.'".  "*  i*"^f?=  S?'  ''«''^"''»  of  Colli  ''*.  dnrsal  column;  A,  dorwl 
-°Hfa  )  ^'"P''"'  "*■  °™'  *•""""';  'f-  ™>f-pl«te;  1-*,  ventral  column;  ..,,  vmtral  zone. 

ment  proceeds,  creeps  dorsally  over  the  surface  of  the  dorsal  horn 
until  it  meets  the  lateral  surface  of  the  fasciculus  of  Goll,  and,  its 
further  progress  toward  the  median  line  being  thus  impeded,  it 
insinuates  itself  between  that  fasciculus  and  the  posterior  horn'  to 
form  thefunicutus  cunealus  {fasciculus  of  Burdach)  (Fig.  23i.B.<:B). 

LitUe  definite  is  as  yet  known  concerning  the  development  of  the 
other  fasacuh  which  are  recognizable  in  the  adult  cord,  but  it  seems 
»5 


i 


i 


1 


386  ™ 

c«uto  tut  the  Uterd  M.d  Mterior  «f»>^.'^'j^'^ 
.T.  »fmMc<l  of  fiben  whlth  erow  downward  in  the  metbet  01  tne 
^r^T^mf^m  neu^btaeS^tueted  in  the  ce«bH^  ~J^',7t 
ih*  rarebeUo-iDinal  (direct  cerebdUr)  fMdcuU  end  the  fiben  of  tne 
f™urd"unS!.?  We  their  origin  horn  cell.  oJ  the  n«ntle  Uyer  oJ  the 

"''^ihe  Bvelinauon  of  the  fiber,  of  the  vtaiJ  ""'..•"•^^'^iXh^'. 
fif.h'^d  Zth  n^nth.  «d  .ppe.™  fij,t  ^n  *«j-'te"f:3^'"i:^»' 
The  myelintaon  of  the  great  motor  ptUis, 
the  lateral  and  anterior  cerebro-.pinal  fas- 
ciculi,!, the  la.t  to  develop,  appearing  to- 
ward the  end  of  the  ninth  month  of  fetal 
life. 

The  Developmwit  of  the  Brain. 

'  —The  enlargement  of  the  anterior 

portion  of  the  medullary  canal  does 

not  take  place  quite  uniformly,  but  is 

less  alonR  two  transverse  lines  than  else 

where,  so  that  the  brain  region  early 
becomes  divided  into  three  primary 
vencles  which  undergo  further  differ- 
enUation  as  follows.    Upon  each  side 
of  the  anterior  vesicle  an  evagination 
appeals  and  becomes  converted  into  a 
club-shaped  structure  attached  to  the 
ventral  portion  of  the  vesicle  by  a 
pedicle.    These    evaginations    (Fig. 
232.  op)  are  known  as  the  optic  euag- 
uuUions,  and  being  concerned  in  the 
formation  of  the  eye  will  be  considered 
in  the  succeeding  chapter.    Alter  thoir 
formation  the  antero-laterai  portions 
of  the  vesicle  become  bulged  out  into  two  P™t«^^^=f^J^  which 
rapidlv  increase  in  size  and  give  rise  eventually  to  Ae  two  '^^«   J 
kLispheres,  which  form,  together  with  *«  PO^'""  "1*^ '"^  ,^ 
which  iL  between  tnem,  what  is  termed  the  uUnuphdon  orfn 
brain,  the  remainder  of  the  vesicle  giving  rise  to  what  is  known  as 


Fio.  132.— RicoNsniocnoii  or 

THS  BlAIN  OF  AN  EUaYO  OI  1.1$ 
KH. 

»,  Hemispheie;  «,  hthmus;  , 
mesencephalon;  «/,  mid-br«in  flex- 
ure; mt,  metencephalon;  myl,  myel- 
encephalon; «/.  ■>»?«  f"""'.  f .  "" 
capsule;  op,  optic  cv»gination;  (, 
diencephalon.— (H«.) 


THt  BIAM  ,gy 

the  a«ne*fh4tltm  or  •twe*H-l>raim  (Fig.  131, 1).  The  middle  vetide  U 
bodily  converted  into  the  mtstnutkaUm  or  mU-hrain  («,),  but  the 
potterior  vesicle  differentiate*  10  that  three  part*  may  be  recognized: 
fi)  a  rather  narrow  portion  which  immediately  succeed*  the  mid- 
brain and  is  termed  the  islhmts  (i);  (j)  a  portion  whose  roof  and 
floor  give  rise  to  the  cerebellum  and  pons  respectively,  and  which  to 
tenned  the mettneefhohn  or hirdbrum  (mf); and  (3)  a  terminal  por- 
tion which  is  known  as  the  medulla  oUtmgala,  or,  to  retain  a  con- 
sistent nomenclature,  the  mytlmcephalon  or  afttr-bram  (my).  From 
each  of  these  six  divisions  definite  structures  arise  whose  relations 
to  the  secondary  divisions  and  to  the  primary  vesicles  may  be  un- 
derstood from  the  following  table  and  from  the  annexed  figure  (Fig. 
n3)<  which  represents  a  median  longitudinal  section  of  the  brain 
of  u  fetus  of  .hree  months. 

My«len«ph«loo  Medull«  oblon|mU  (I). 


jid  Voicle  . 


Metenceplulon 


lithmui 


f  Pom  (II  I) 


ind  Voicle. 


McKDCephmloit 


■stVedde. 


Diencephalon 
Telencephmlon 


CerebeUum  (11 1). 


Bnchia  coojunctiva  (III). 
.Cerebral  peduncles  (poitcri 
portion). 

(Cerebral  peduncle!  (anterior  por- 
tion) (IV  I). 
Corpora  quadrigemina  (IV  a). 

(Part  manunillarja  (V  l). 
Thalamus  (V  a). 
Epiphyiis  (V  3). 

f  Infundibulum  (VI  i). 
)  Corpus  striatum  (VI 1). 
I  Olfactory  bulb  (VI  3). 
I  Hemispheres  (VI  4). 


But  while  the  walls  of  the  primary  vesicles  undergo  this  complex 
differentiation,  their  cavities  retain  much  more  perfectly  their 
nnginal  relations,  only  that  of  the  first  vesicle  sharing  to  any  great 
extent  the  modifications  of  the  walls. 


II  \\ 


-gg  THE   BRAIN 

The  cavity  of  the  third  vesicle  persists  in  the  adult  as  tiie  fourth 
venlricle,  traversmg  all  the  subdivisions  of  the  vesicle;  that  of  the 
second,  increasing  but  little  in  height  and  breadth,  constitutes  the 
aquaducius  cerebri;  while  that  of  the  first  vesicle  is  continued  into 
the  cerebral  hemispheres  to  form  the  lateral  ventricles,  the  remamder 
of  it  constituting  the  third  ventricle,  which  includes  the  cavity  of 
the  median  portion  of  the  telencephalon  as  well  as  the  entire  cavity 
of  the  diencephalon.  ,   .     v    • 

During  the  differentiation  of  the  various  divisions  of  the  brain 
certain  f  exurcs  appear  in  the  roof  and  floor,  and  to  a  certain  extent 


F,0    „3.-MeD«N  LONOnUDINAl  SECTION  OF  THE  B.U.N  OF  AN  E«B.VO  or  THE 

"       •'•'  Third  Month.— (His.) 

correspond  with  those  already  described  as  occurring  in  the  embryo. 
The  first  of  these  flexures  to  appear  occurs  in  the  region  of  the  mid- 
brain, the  first  vesicle  being  bent  ventrally  until  it  comes  to  lie  at 
practically  a  right  a..gle  with  the  axis  of  the  mid-brain.  This  may 
be  termed  the  mid-brain  flexure  (Fig.  232,  mf)  and  corresponds  wuh 
the  head-bend  of  the  embryo.  The  second  flexure  occurs  in  the 
region  of  the  medulla  oblongata  and  is  known  as  the  nape  flexure 
(Fig  232  «/);  it  correRponds  with  the  similarly  named  bend  of  tht 
embryo  and  is  produced  by  a  bending  ventrally  of  the  entire  head,  so 


THE    IfYELENCEEHAlON  ^g,, 

S  !l!'»'^'u°*  ""'  ™''^-''™'"  ^"-"^^  '°  "^  "''"o^'  ^t  right  angles 

and  IS  „t.rely  peculmr  to  the  nervous  system;  it  consists  of  abending 
ve„  rally  of  he  floor  of  the  hind-brain,  the  roof  of  this  portion  of  the 

.hTS'^c'r.^;^ "'  ^" '  -^^  '"''^-'''- '"-  - 

oJZ  lo'tle  h'  '^7''°P"'^"'  "'^  P°"^  fl""^e  practically  disappears. 

nl  lei  of  IT"'  i"  "■'  '"^«'°"  °'  '"*  '^^"-''^"^  fibers  and 
nucle.  of  the  pons,  but  the  mid-brain  and  nape  flexures  persist 

If  ShTk'  "t""'  '"  ''^""=""''  '"^  ^«^  of  'he  anterior  S 
of  th  adult  bram  be.ng  inclined  to  that  of  the  medulla  at  an  angle  of 
about  134  degrees.  ^ 

The  Developmen,  of  the  Myelencephalon.~ln  its  posterior  portion 

fmiUr  r'T      °"  '^'""'^  ^^^^'''^^  '"^  ^P-^'  ^-d  and  has  a  very 

fZ  L.  :°"";"'  '"'"  '"'^"°^'^'  ''°'^--'  'he  roof-pla^ 
^■g.  m,  rp)  widens  to  form  an  exceedingly  thin  membrane   the 

datl"  r   !;'  ""  ""=  "■°'''""«  °f  ">«  ^-f-P''"-  'here  is  as  so 
S      H  ■;'  °'  ''^  '*°"^'  P°"'°"  °f  'he  brain  cavity,  the 

do^l  and  ventral  zones  bending  outward,  until,  in  the  am  rio 
portK,n  of   he  after-brain,  the  margins  of  the  dorsal  zone  have  a 

ZtTT\  "t.'"'  "'"'•  '^"'  ^'^"'^^"^  '"  f°™  -  ^fleeted 
Lh- ^"  ^'''/^u  u  '  P°'"°"  °f  "'^  f°"«h  ventricle  contained  in 
a4vrr'"?''^'^°'""'''"^™"-"«»'"'-hroad  shallow 
cavity,  whose  floor  IS  formed  by  the  ventral  zones  separated  in  the 
m  d.an  hne  by  a  deep  groove,  the  floor  of  which  is  the  somewha 

roof-plate  a  transverse  groove  into  which  the  surrounding  mesen- 
chyme d.ps  and  as  the  groove  deepens  in  later  stages,  the  mesen- 
hyme  contamed  withm  it  becomes  converted  into  blood-vessels 
ormmg  the  chorundpU^^s  of  the  fourth  ventricle,  a  structure  which,' 
.1   may  be  seen  from  Us  development,  does  not  lie  within  the  cavitJ 

Lr/JK   w    '  ''"' ''f  P""'"*  ^^^  ''  by  the  portion  of  the  roof'- 
'late  which  forms  the  floor  of  the  groove. 

In  embryos  of  about  9  mm.  the  differentiation  of  the  dorsal 


\\     \ 


-go  THE   MYELENCEFHALON 

and  ventral  zones  into  ependymal  and  mantle  layers  is  clearly  visible 
(Fig.  234),  and  in  the  ventral  zone  the  marginal  velum  is  also  well 
developed.  Where  the  fibers  from  the  sensory  ganglion  of  the  vagus 
nerve  enter  the  dorsal  zone  an  oval  area  (Fig.  234, /s)  is  to  be  seen 
which  is  evidently  comparable  to  the  oval  bundle  of  the  cord  and 
consequently  with  the  fasciculus  of  Burdach.  It  gives  rise  to  the 
solitary  fasckultts  of  adult  anatomy,  and  in  embryos  of  11  to  13  mm. 
it  becomes  covered  in  by  the  fusion  of  the  reflecltd  lip  of  the  dorsal 
zone  with  the  sides  of  the  myelencephalon,  this  fusion,  at  the  same 
time,  drawing  the  margins  of  the  roof-plate  ventrally  to  form  a 


Fio.  234.— TiANSVEUSE  Section  thiiouoh  the  MEDOtiA  Oblongata  of 

AN  EUBRYO  OF  9.1  UU. 

ds  Dors.1  «>ne;ft,  floor-pUte;/!,  fasciculus  solitarius;  (,  lip:  r/.  roof-platt;  m,  ventra! 
ione;  X  and  Xtt,  tenth  and  twelfth  nerves.— (H«i.) 

secondary  lip  (Fig.  235).  Soon  after  this  a  remarkable  migration 
ventrally  of  neuroblasts  of  the  dorsal  zone  begins.  Increasing 
rapidly  in  number  the  migrating  cells  pass  on  either  side  of  the  soli 
tary  fasciculus  toward  the  territory  of  the  ventral  zone,  and,  passin'.; 
ventrally  to  the  ventral  portion  of  the  mantle  layer,  into  which 
fibers  have  penetrated  and  which  becomes  the  formatio  reticularis 
(Fig.  235, »,  they  differentiate  to  form  the  olivary  body  (ol). 

The  thickening  of  the  floor-plate  gives  opportunity  for  fibers  to 
pass  across  the  median  line  from  one  side  to  the  other,  and  this 
opportunity  is  taken  advantage  of  at  an  early  stage  by  the  axis-cylir- 


IHE   MnUNCEFRALON  -gj 

^I™  H  '^.  ^^'  °*'^'  fi*^"'  descending  f^m  the  c^e- 

nSL     of  the  'f',''  ij'^  "'  ""=  ""^""^  fi"^"  f«""  the 
neuroblasts  of  the  nuclei  gntclis  and  cuneatus,  which  seem  to  be 


fn  Sl^'*  '^'^  't"  r  ""'=  ""'^^  °^  """^ '»°"«'  «'°e.  '^ko  decussate 
m  the  substance  of  the  floor-plate;  these  fibers,  known  as  the  arZ  ! 
fibers,  pass  in  part  to  the  cerebeUum,  associlting  ther^selves  S 
fibers  ascending  from  the  spinal  cord  and  with  the  ohW  fiU  o 

velum  and  known  as  the  restiform  body  (Fig.  235  tr) 

The  principal  differentiations  of  the  zones  of  the  myelencephalon 
may  be  stated  in  tabular  form  as  follows:  ^eiencepnalon 

'*°°'-^■»'' Posterior  vdum. 

■-'»'»«« fS^LrTdt.^'.r'^™'''^^"^--- 

[  The  oliraiy  bodies. 

Vmlrml  zones /  ^j)"^'' ?* ""«™  "f  *«  notor  roots  of  cranW  nerves. 

^  The  reticukr  fonnatioa. 
r-piate tj^  mediin  raphe. 


;!■    ill 
I       I 


If     1! 


II  1 


39» 


THE   CEKEBEILUII 


The  Development  of  the  Metencephulonand  /irtmiu.— Our  knowl- 
edge of  the  development  of  the  metencephalon,  isthmus,  and  me?  .n- 
cephalon  is  by  no  means  as  complete  as  is  that  of  the  myelencephalon. 
The  pons  develops  as  a  thickening  of  the  portion  of  the  brain  floor 
which  forms  the  anterior  wall  of  the  pons  flexure,  and  its  transverse 
fibers  are  well  developed  by  the  fourth  month  (Mihalkovicz),  but  all 
details  regarding  the  origin  of  the  pons  nuclei  are  as  yet  wanting. 
If  one  may  argue  from  what  occurs  in  the  myelencephalon,  it  seems 
probable  that  the  reticular  formation  of  the  metencephalon  is  derived 
from  the  ventral  zone,  and  that  the  median  raphe  represents  the 
floor-plate.  Furthermore,  the  relations  of  the  pons  nuclei  to  the 
reticular  formation  on  the  onb  hand,  and  its  connection  by  means  of 


Fio  i\6—A  DoKSAL  View  OF  THE  Biain' OF  A  Rabbit  Embryo  or  i6ii«.;B,MiDiAS 
■  I^NomroiNAL  Section  of  a  Calf  Embeyo  of  3  --m. 

c.  Cerebellum;  m,  mid-brain.— (Mi*<i)*ow«) 

the  transverse  pons  fibers  with  the  cerebellum  on  the  other,  suggest 
the  possibility  that  they  may  be  the  metencephalic  representatives 
of  the  olivary  bodies  and  are  formed  by  a  migration  ventrally  of 
neuroblasts  from  the  dorsal  zones,  such  a  migration  having  been 
observed  to  occur  (Essick). 

The  cerehellum  is  formed  from  the  dorsal  zones  and  roof-plate 
of  the  metencephalon  and  is  a  thickening  of  ;he  tissue  immediately 
anterior  to  the  front  edge  of  the  posterior  velu  -a.  This  latter  struc 
ture  has  in  eariy  stages  a  rhomboidal  shape  (Fig.  336,  A)  which 
causes  the  cerebellar  thickening  to  appear  at  first  as  if  composed 
of  two  I  -teral  portions  inclined  obliquely  toward  one  another.  In 
reality,  however,  the  thickening  extends  entirely  across  the  roof  of 


THE    CEREBEltCM 

late  structure  characteristic  of 
the  cer>,.)ellum. 

The  histogenetic  develop- 
ment of  the  cerebellum  at  first 
proceeds  along  the  lines  which 
have  already  been  described 
as  typical,  but  after  the  devel- 
opment of  the  mantle  layer  the 
cells  lining  the  greater  portion 

of  the  cavity  of  the  ventricle  <». — -  ^  ( 

cease  to  multiply,  only  those  „F'°-  :>37.-DiAr,RAM  Rep,esfvt.no  the 
which  are  situated  in  the  roof-  °'I™^.^""'™""«Cebebe.:I';2el'^ 
plate  of  the  metencephalon  ^''-'^P^^'«^rZ^^Lr'S,J^, 
and  along  the  line  of  junction  ^"5^^?-^^^^.,::"^^  ^riS 
of  the  cerebellar  thickening  L^,^.' ''^' -Pf-l™  f"r™w,andx;ffeor<5TK 
with  the  roof-plate  continuing  -"''■*-' "^^-MO 

to  divide     The  indifferent  cells  formed  in  these  regions  migrate 
outward  from  the  median  line  and  forward  in  the  mTrXul 

and  cover  he  entire  surface  of  the  cerebellum  (Fig.  .„)  The 
el  s  of  ths  layer,  like  those  of  the  mantle,  differeu.iate  into  n  urollt 
■Is  and  neuroblasts,  the  latter  for  the  most  part  migrating  cemrf! 
at  a  later  stage  to  mingle  with  the  cells  of  the  manl^e  layer  aSt« 
become  transformed  into  the  granular  cells  of  the  cerebelLr  cortel 
The  neurogha  cells  remain  at  the  surface,  however,  forming  the 
jncf.  consntuent  of  the  outer  or,  as  it  is  now  termed  the  .21 
layer  of  the  cortex,  and  mto  this  the  dendrites  of  the  Purkinje  cells 


394 


THE  ISTHMUS 


probably  derived  from  the  mantle  layer,  project.  The  «n»g^^»° 
of  the  neuroblasts  of  the  epithelUl  layer  U  probably  comp  eted 
before  birth,  at  which  time  but  few  remain  in  the  molecular  layer 
to  form  the  stellate  cells  of  the  adult.  The  origin  of  the  dentate  and 
other  nuclei  of  the  cerebellum  is  at  present  unknown,  but  it  seems 
probable  that  they  arise  from  cells  of  the  mantle  layer. 

The  nerve-fibers  which  form  the  medullary  substance  of  the 
cerebellum  do  not  make  their  appearance  until  about  the  siith 
month,  when  they  are  to  be  found  in  the  ependymal  tissue  on  the 
inner  surface  of  the  layer  of  granular  cells.    Those  which  are  nc 
commissural  or  associative  in  function  converge  to  the  line  of  junction 
of  the  cerebellum  with  the  poi»s,  and  there  pass  into  the  marginal 
velum  of  the  pons,  myelencephalon,  or  isthmus  as  the  case  may  be. 
The  dorsal  surface  of  the  isthmus  is  at  first  barely  disUnguishable 
from  the  cerebellum,  but  as  development  proceeds  its  roof-plate 
undergoes  changes  similar  to  those  occurring  in  the  medulla  ob- 
longata and  becomes  converted  into  the  anterior  velum.    In  the 
dorsal  portion  of  its  marginal  velum  fibers  passing  to  and  from  the 
cerebellum  appear  and  form  the  brachia  conjunctiva,  while  ventrally 
fibers,  descending  from  the  more  anterior  portions  of  the  brain,  form 
the  cerebral  peduncles.    Nothing  is  at  present  known  as  to  the  history 
of  the  gray  matter  of  this  division  of  the  brain,  although  it  ma.y  be 
presumed  that  its  ventral  zones  take  part  in  the  formation  of  the 
tegmentum,  while  from  its  dorsal  zones  the  nuclei  of  the  brachia  con- 
junctiva are  possibly  derived.  ,   ^   ^   „  „( 
The  following  table  gives  the  origin  of  the  principal  structures  of 
the  metencephalon  and  isthmus: 


Metencepkalom  ■ 


Rxx>{-pUte 


Donal  tones. 


r  Posterior  velum. 

I  Vermis  of  cerebellum. 

Lobes  of  cerebellum. 
Flocculi. 

Nuclei  of  tenmnalion  of  sen- 
sory roots  of  cranial  nerves. 
Pons  nuclei. 


ISIHUCS. 

Anterior  velum. 
Brachia  conjunctiva. 


IHE    MESENCEPHALON  jg, 

MITINC.PHAU,„.  ,,^^^^ 

V.ntnilzon«  f  N-'W  of  origin  of  motor    Po.t.rior    part    o(     „r,hr.l 

ventral  zones roots  of  cranial  nerves.  peduncles 

Floor-plat.  ^  Mtli""'"  'T'"™-  ^°'"^'  P»"  "'  "Kmentum 

P'" M<^'"n  raphe.  Median  raphe. 

The  Development  of  the  Meser,cephalon.-Our  knowledge  of  the 
development  of  this  portion  of  the  brain  is  again  very  imix-rfect 
Dunng  the  stages  when  the  flexures  of  the  brain  are  well  mnrked 
(*igs.  232  and  233)  it  forms  a  very  prominent  structure  and  pos- 
sesses for  a  fme  a  capacious  cavity.     Later,  however,  it  increases  in 
su!e  less  rapidly  than  adjacent  parts  and  its  walls  thicken,  the  roof- 
and  floor-plates  as  well  as  the  zones,  and,  as  a  result,  the  caWty 
becomes  the  relatively  smaller  canal-like  cerebral  aqu^uct      In  the 
marginal  velum  of  its  ventral  zone  fibers  appear  at  about  the  third 
month,  forming  the  anterior  portion  of  the  cerebral  peduncles,  and, 
at  the  same  time,  a  median  longitudinal  furrow  appears  upon  the 
dorral  surface,  dividing  it  into  two  lateral  elevations  which,  in  the 
hfth  month,  are  divided  transversely  by  a  second  furrow  and  are 
hus  converted  from  corpora  bigemina  (in  which  form  they  are 
found  in  the  lower  vertebrates)  into  corpora  quadrigemina. 

Nothing  is  known  as  to  '-.e  differentiation  of  the  erav  matter  nf  th. 
do™l  and  ventral  zones  of  tne  mid-brain.    Fr<n^  the  r&H  the  narts 

nt^tn"^  developtnents  of  the  dorsal  zones,  as  may  also  b?Se 

mayVe^ta'Sus-"''  representing  the  origin  of  the  mid-brain  structures 

Roof-plate ,-, 

Dorsal  lones /  Corpora  quadrigemina. 

\  Red  nuclei. 

,,         ,  f  f*"":'''  of  origin  of  the  third  and  fourth  nerves 

Ventral  zones •  .interior  pan  of  tegmentum. 

[  Anterior  part  of  cerebral  peduncles. 
*^'-pl«te Median  raphe. 


W  '^1 


396 


THX  DIENCEIHAION 


The  Devehpment  of  the  Diencethahm.-K  transverse  section 
through  the  diencephalon  of  an  embryo  of  about  five  weeks  (Fig. 
238)  shows  clearly  the  differentiation  of  this  portion  of  the  brain  into 
the  typical  zones,  the  roof-plate  (r^)  being  represented  by  a  thin- 
walled,  somewhat  folded  area,  the  floor-plate  (/?)  by  the  tissue 
forming  the  floor  of  a  well-marked  ventral  groove,  while  each  lateral 
wall  is  divided  into  a  dorsal  and  ventral  zone  by  a  groove  known  as 
the  sulcus  Monroi  (5m),  which  extends  forward  and  ventrally 
toward  the  point  of  ongin  of  the  optic 

Oevagination  CP'g-  240)-    At  the  pos- 
terior end  of  the  ridge-like  elevation 
whitn  represents  the  roof-plate  is  a 
rounded  elevation  (Fig.  239,  P)  which, 
in  later  stages,  elongates  until  it  al- 
"  most  reaches  the  dermis,  forming  a 
hollow  evagination  of  the  brain  roof 
known  as  the  pineal  process.    The  dis- 
tal extremity  of  this  process  enlarges  to 
a  sac-like  structure  which  later  be 
F.o.,38.-T«Nsv.««  SECTION    comes  lobed,  and,  by  an  active  pro- 
Of  TBI  Diencephalon  or  an  Em-    Hferation  of  the  cells  lining  the  cavi- 
beyoofFiveWeees.  ,  ^j  jj^^  ^^^^^^  ,„bes,  finally  be 

dt   Dorsal  zone     fp,  door-plate;  ^i.    j. .       i  k  J*< 

rf,  roof-plate;  Sm,    ileus  Monroi;    comes  a  solid  structure,  the  pineal  OOC) 
«;  ventral  a)ne.—(>.rf.)  -j-j^^  j^^^g  proximal  portion  of  thi' 

evagination,  remaining  hollow,  forms  the  pineal  stalk,  and  the  en- 
tire structure,  body  and  stalk,  constitutes  what  is  known  as  tho 
epiphysis. 

The  significance  of  this  organ  in  the  Mammalia  is  doubtful.  In  the 
ReptiUa  and  other  lower  forms  the  outgrowth  is  double,  a  secondary 
outgrowth  arising  from  the  base  or  from  the  anterior  wall  of  the  primary 
one  This  anterior  evagination  elongates  ui.ui  it  reaches  the  dorsi 
epidermis  of  the  head,  and,  there  expanding,  develops  into  an  unpairwl 
eye  the  epidermis  which  overlies  it  becoming  converted  into  a  trar- 
parent  cornea.  In  the  MammaUa  this  anterior  process  does  not  develop 
and  the  epiphysis  in  these  forms  is  comparable  only  to  the  posteri  >r 
process  of  the  Reptilia.  , 

In  addition  to  the  epiphysial  evaginations,  another  evagination  ari-ts 


^I,f 


THE   DIENCEPHAION  _jg^ 

present  has  not  been  found  in  other 
groups  of  the  Mammalia.  It  seems  to 
be  comparable  to  a  chorioid  plexus 
which  IS  evaginated  from  the  brain 
surface  instead  of  being  invaginated 
MIS  usually  the  case.  There  is  no  evi- 
dence that  a  paraphysis  is  developed  <-p 
m  the  human  brain. 

The  portion  of  the  roof-plate 
which  h'es  in  front  of  the  epiphysis 
represents   the  velum  inierposilum 
of  the  adult  brain,  and  it  forms  at 
firjt  a  distinct  ridge  (Fig.  239,  rp). 
At  an  early  stage,  however,  it  be- 
come, reduced  to  a  thin  membrane 
upon  the  surface  of  which  blood- 
vessels, Qiveloping  in  the  surround- 
ing   mesanhyme,   arrange   them- 
selves at  about  the  third  month  in 
two  longitudinal  plexuses,  which, 
with  the  subjacent  portions  of  the 
velum,  become  invaginated  into  the 
cavity  of  the  third  ventricle  to  for-i    BiSrv  '■^~«^^'-  ^"=''  "'  ™= 


tMBJlYO  or  13.6  UH. 

».  Superior  brachium;  eg,  lateral 
geniculate  body;  cp,  chorioid  plexus 
i?o,  anterior  corpus  quadrigeminum; 
*,  hippocampus;  A/,  hippocampal  fis- 
sure; «t,  thalamus;  p,  pineal  body;  rp, 
roof-plate.— (Hij.)  '     '^' 


The  dorsal  zones  thicken  in 
their  more  dorsal  and  anterior 
portions  to  form  massive  structures, 
the  thahmi  (Figs.  233,  V2,  and 
239.  "0.  which,  encroaching  upon 

space,  so  narrow,  mdeed,  that  at  about  the  fifth  month  the  inner 

urfaces  of  the  two  thalami  come  in  contact  in  the  median  Tine 

forming  what  is  known  as  the  in,ermeiiale  nurs.    MoreventX 


398 


TBB  TXURCBPHALaM 


^j 


and  posteriorly  another  thickening  of  the  dorsal  rone  occur.,  ghnng 
rise  oTeach  side  to  the  pulvinar  of  the  thalamus  and  »»  »  W«,^ 
^::M  My,  and  two  ridges  extending  Uckward  and  dorsaUy 
km  the  latter  structures  to  the  thickenmg.  m  the  roof  of  the  mid- 
brain which  represent  the  anterior  corpora  'l»«'"f '"'"f '  «^ 
path  along  which  the  nerve-fibers  which  constitute  the  supenor 
ouadritemiual  brachia  pass.  ... 

From  the  ventral  zones  what  is  known  as  the  hypolka^mtc  regum 
develops,  a  mass  of  fibers  and  cells  whose  relations  and  development 
are  noVyet  clearly  understood,  but  which  may  be  r^<^f^^^ 
forward  continuation  of  the  tegmentum  and  >^«t'="l»'  »™»''°"; 
In  the  median  Une  of  the  floor  of  the  ventricle  an  ""P^.'^^  jhickemng 
appears,  representing  the  corpora  mamiUana.  which  dunng  the 
thWm^nth  becomes  divided  by  a  median  furrow  into  two  rounded 
eminences;  but  whether  these  structures  and  the  postenor  portion 
of  the  tuber  cmcreum,  which  also  develops  from  this  region  of  the 
brain,  are  derivatives  of  the  ventral  zones  or  of  the  floor-plate  is  as 
yet  uncertain. 

Assuming  that  the  mamillaria  and  the  tuber  'i'"?""' "'^^^'''.f^ 
froXTe^ntral  zones,  the  O"?"' »«  ^V'^X"^.'""^  '"""  '^' 
walls  of  the  diencephalon  may  be  tabulated  as  foUows. 


f  Velum  interpoutum. 
\  Epiphysis. 
ThalamL 
Pulvinarei. 

Lateral  geniculate  bodies. 
Hypothalamic  r^ion. 
Corpora  mamillaria. 
Tuber  dnereum  (in  part). 
Floor-plate Tissue  of  mid-ventral  Une. 


Roof-plate  . 


Dorsal 


Ventral  xmes. 


The  Development  of  the  Telencephdon.-Vor  convenience  of 
description  the  telencephalon  may  be  regarded  as  consistii^  of  .i 
median  portion,  which  contains  the  anterior  part  of  the  third  ven- 
tricle and  two  lateral  outgrowths  which  constitute  the  cerebral 
hemispheres.  The  roof  of  the  median  portion  undergoes  the  same 
transformation  as  does  the  greater  portion  of  that  of  the  diencephalo. 


TBI  RIXNCXPHALOM 


399 


Md  is  converted  , mo  the  anterior  part  of  the  velum  interpositum 
(Fig.  340.  vi).  Anteriorly  this  passes  into  the  anterior  waU  of  the 
third  ventncle,  the  lamina  UrminalU  (U).  a  structure  which  is  to  be 
r^arded  as  formed  by  the  union  of  the  dorsal  zones  of  opposite 
«des,  since  It  Ues  entirely  dorsal  to  the  anterior  end  of  the  sulcus 
Monro..  From  the  ventral  par  of  the  dorsal  zones  the  optic 
evaginations  are  formed,  a  depression,  the  optic  recess  {or),  marking 
their  point  of  origin. 

The  ventral  zones  are  but  feebly  developed,  and  form  the  anterior 
part  of  the  hypothalamic  region,  while  at  the  anterior  extremity 
of  the  floor-plate  an  evagination  occurs,  the  infundibular  recess  lir), 
which  elongates  to  form  a  funnel-shaped  structure  known  as  the 

Kf  ^r^u'  '",  "'"'""■'^  ""^  ^yWhysis  comes  in  contact 
during  the  fifth  week  with  the  enlarged  extremity  of  Rathke's  pouch 
fomed  by  an  invagination  of  the  roof  of  the  oral  sinus  (see  p.  285), 
and  apphes  itself  closely  to  the  posterior  surface  of  this  (Fig.  j,, 
to  form  with  it  the  pituUary  body.    The  anterior  lobe  at  an  early 
stage  separates  from  the  mucous  membrane  of  the  oral  sinus,  the 
Stalk  by  which  It  was  attached  completely  disappearing,  and  toward 
the  end  of  the  second  month  it  begins  to  send  out  processes  from 
Its  waUs  into  the  surrounding  mesenchyme  and  so  becomes  con- 
verted  into  a  mass  of  solid  epithelial  cords  embedded  in  a  mesen- 
chyme nch  m  blood  and  lymphatic  vessels.    The  cords  later  on 
divide  transversely  to  a  greater  or  less  extent  to  form  alveoli,  the 
entire  structure  coming  to  resemble  somewhat  the  parathyieoid 
bodies  (see  p.  297),  and,  like  these,  having  the  function  of  producing 
an  internal  secretion.    The  posterior  lobe,  derived  from  the  brain 
retains  its  connection  with  that  structure,  its  stalk  being  the  infun- 
atMum,  but  Its  terminal  portion  does  not  undeigo  such  extensive 
modifications  as  does  the  anterior  lobe,  although  it  is  claimed  that 
It  gives  nse  to  a  glandular  epithelium  which  may  become  arranged 
so  as  to  form  alveoli. 

The  cerebral  hemispheres  ar.  formed  from  the  lateral  portions 
"  the  dorsal  zones,  each  possessing  also  a  prolongation  of  the  roof- 
piate.    From  the  more  ventral  portion  of  each  dorsal  zone  there  is 


400 


TKr.   T«I.ENCE1>HA10N 


formed  a  thickening,  the  corpus  striatum  (Figs.  a40,  cs,  and  233,  VI  t), 
a  structure  which  is  for  the  telencephalon  what  the  optic  thalamus 
is  for  the  diencephalon,  while  from  the  more  dorsal  portion  Inere  is 
formed  the  remaining  or  mantle  (pallial)  portions  of  the  hemispheres 
(Figs.  a40,  h,  and  tij,  VI  4).  When  first  formed,  the  hemispheres 
are  slight  evaginations  from  the  median  portion  of  the  telencephalon, 
the  openings  by  which  their  cavities  communicate  with  the  third 
irentricle,  the  intervtntrkular  foramina,  being  nlatiytAy  very  large 
(Fig.  140),  but.  In  later  stages  (Fig.  :ii),  the  hemispheres  increase 
more  markedly  and  eventually  su' pass  all  the  other  portions  of  the 


Fio.  340.— Medjin  LoNomjDiNAL  Section  of  the  B«ain  o»  an  Ekbeyo  or  16.3  KM- 
br,  Anterior  brachium;  eg,  corpus  geniculatum  laterale;  cs,  corpus  striatum;  h, 
cerebral  hemisphere;  i>,  infundibular  recess;  //,  lamina  lerminalis;  or,  op''  recess;  o(, 
thalamus;  p,  pineal  process;  sm,  sulcus  Monroi;  5/,  hypothalamic  region;  vi,  velum 
interpositum. — (His.) 

brain  in  magnitude,  overlapping  and  completely  concealing  the 
roof  and  sides  of  the  diencephalon  and  mesencephalon  and  also  the 
anterior  surface  of  the  cerebellum.  In  this  enlargement,  however, 
the  interventricular  foramina  share  only  to  a  slight  extent,  and 
consequently  become  relatively  smaller  (Fig.  233),  forming  in  the 
adult  merely  slit-like  openings  lying  between  the  lamina  terminalis 
and  the  thalami  and  having  for  their  roof  the  anterior  portion  of  tht 
velum  interpositum. 

The  velum  interpositum — that  is  to  say,  the  roof-plate— where 


■na  TEUNCEPRAIOK  ^o, 

uolnThVlj'  ""T'  V^'  interventricular  foramen,  i.  prolonged  out 
upon  the  don«l  .ur^ace  of  each  hemisphere,  and,  Joming^vag- 
inated,  form,  upon  i.  a  groove.  As  the  hemispheres,  inc,!^,ing  fn 
S^T  "  '  ■""'"'  *'"■  '"•  f'^^'-  ^hich  is  the  so-c^Ied 
1Z^/''^'.IT:'^''  ""  "'""^  "■«  ^•"""'  '^8'  of  ""•»  wall, 
more  e  „„o*^^r^!  ""^  '»«"^«P''""  ^""'"""^  i'  becomes  more  and 
more  elongated,  be.ng  carried  at  first  backward  (Fig.  ,4,)    then 

After  the  estabhshment  of  the  grooves  the  mesenchymVin  their 

2^T         ',  r-.""'^'  '*'=^"°P'"«  blood-vessels,  becomes  the 
ch^  pluses  of  the  lateral  ventricles,  and  at  first  these  plexusel 

completely.    Uter,  however,  the  walls 

of  the  hemispheres  gain  the  ascendancy 

in  rapidity  of  growth  and  the  plexuses 

become  relatively  much  smaller.     Since 

the  portions  of  the  roof-plate  which  form 

the  chorioidal  fissures   are   continuous 

with  the  velum  interpositum  in  the  roofs 
of  the  interventricular  foramina,  the 
chonoid  plexuses  of  the  lateral  and  third 
ventricles  become  continuous  also  at  that 
point. 

The  mode  of  growth  of  the  chorioid 
fissures  seems  to  indicate  the  mode  of 

growth  of  the  hemispheres.    At  first  the    •■' 

growth  is  more  or  less  equal  in  all  directions,  but  later  it  '^.comes  more 

dS:.:  T„T  t"'  I'T  *"'"«  """"^  '^"^  ^-  -p--"^  "b 

th™  '  ''''«".f"«her  extension  backward  becomes  difficult 

temporal  lobes.  As  a  result  the  cavities  of  the  hemispheres  the 
tri  rTr'"'  ",  ^'"'"°"  '"  '^'"^  ^^^"^  forward  to  f^m  an 
lateraUr  descending  horn,  and  the  corpus  striatum  likewise  extends 


Fio.   341.— .Median    Longi- 

TUDWAtSlCTIO.V  or  THE  BeAIN 

Of  AN  Ehbeyo  Caw  of  5  cu. 

cb.  Cerebellum;  cp,  rhorioid 
plexus;  M,  corpus  striatum;/.)/, 
interventricular  foramen;  in, 
hypophysis;  m,  mid-brain;  oc, 
optic  commissure;  /,  posterior 
PfTt  of  the  diencephilon.— 
(Mihalkm'icz.) 


403 


TBK  IKUEKCEimALON 


backward  to  the  tip  of  each  temporal  lobe  as  a  slender  process  known 
as  the  tail  of  the  caudate  nucleus.  In  addition  to  the  anterior  and 
lateral  horns,  the  ventricles  of  the  human  brain  also  possess  postenor 
horns  extending  backward  into  the  occipital  portions  of  the  hemis- 
pheres, these  portions,  on  account  of  the  greater  persistence  of  the 
mid-brain  flexure  (see  p.  388),  being  enabled  to  develop  to  a  greater 
extent  than  in  the  lower  mammals. 

The  scheme  of  the  origin  of  parts  in  the  telencephalon  may  be 
stated  ai  follows: 


Mediam  Fart. 


Heuispherxs. 


Roof-pUte.. 


r  Anterior  part  of  velum   inter-  f  p^^  ^  chorioidal  fissure. 


positum. 


Pallium. 
Corpus  striatum. 
Olfactory  lobes  (see  p.  406) 


/  Lamina  terminalis. 

Dorsal  zones <  ^^^  evaginations. 

Anterior  part  of  hypothalamic 

Ventral  zones region. 

Anterior  part  of  tuber  cinereura. 

The  Convolutions  of  the  Hemispheres.-The  growth  of  the 
hemispheres  to  form  the  voluminous  structures  found  in  the  adult 
depends  mainly  upon  an  increase  of  size  of  the  pallium.  The 
corpus  striatum,  although  it  takes  part  in  the  elongation  of  each 
hemisphere,  nevertheless  does  not  increase  in  other  du-ections  as 
rapidly  and  extensively  as  the  pallium,  and  hence,  even  in  very  early 
stages,  a  depression  appears  upon  the  surface  of  the  hemispheres 
where  the  corpus  is  situated  (Fig.  242)-  This  depression  is  the 
lateral  cerebral  fossa,  and  for  a  considerable  period  it  is  the  only  sign 
of  inequality  of  growth  on  the  outer  surfaces  of  the  hemispheres. 
Upon  the  mesial  surfaces,  however,  at  about  the  time  that  the 
choroid  fissure  appears,  another  linear  depression  is  formed  doroal 
to  the  chorioid,  and  when  fully  formed  extends  from  in  front  of  the 
interventricular  foramen  to  the  tip  of  the  temporal  lobe  (Fig.  244>  h  ■ 
It  affects  the  entire  thickness  of  the  pallial  wall  and  consequently 
produces  an  elevation  upon  the  inner  surface,  a  projecUon  into  tie 
cavity  of  the  ventricle  which  is  known  as  the  hippocampus,  when,  e 


THE  CEKEBSAL  CONVOLDTIONS 


403 


tl  Sr  ■"?•  ^■'"""'^  *•'''  MppocampalAssure.    The  portion  of 
forms  what  is  known  as  the  dentate  gyi  i 

two  prolongafons  anse  from  the  fissur..  j.st  ^vher.  it  turns  to  be 
contmued  mto  the  temporal  lobe,  and  these,  extending  posteriorly 
g.ve  nse  to  the^a„>,„-<,„i^,„,  and  calcarine  fissures.  Like  the 
h.ppocampal,  these  fissures  produce  elevations  upon  the  inner 
surface  of  the  pallium,  that  formed  by  the  parieto-^cipiul  ear^ 
disappearing,  while  that  pro-  ^ 

duced  by  the  calcarine  persists 
to  form  the  calcar  {hippocam- 
pus minor)  of  adult  anatomy. 
The    three   fissures   just 
described,  together  with  the 
chorioidal   and    the    lateral 
cerebral  fossa,  are  all  formed 
by  the  beginning  of  the  fourth 
month   and  all  the  fissures 
affect  the  entire  thickness  of 
the  wall  of  the  hemisphere, 
and  hence  have  been  termed 
the  primary  or  total  fissures. 
Until  the  beginning  of  the  fifth 
month  they  are  the  only  fissures  present,  but  at  that  time  secondary 
fassures,  which,  with  one  exception,  are  merely  furrows  of  the  sur- 
face of  the  pallium,  make  their  appearance  and  continue  to  form 
until  birth  and  possibly  later.    Before  considering  these,  however, 
certain  changes  which  occur  in  the  neighborhood  of  the  lateral 
cerebral  fossa  may  be  described. 

The  fossa  is  at  first  a  triangular  depression  situated  above  the 
temporal  lobe  on  the  surface  of  the  hemisphere.  During  the  fourth 
.nonth  It  deepens  considerably,  so  that  its  upper  and  lower  margins 
-ccome  more  pronounced  and  form  projecting  folds,  and,  during 
the  fifth  month,  these  two  folds  approach  one  another  and  eventually 


Fro.  341.— BuiNOr  an  Eiibrvo  or  th« 

Fourth  Month. 

c,  Cerebellum;  p,  pons;  j,  lateral  cerebral 

fossa. 


TEE  CEREBRAL  CONVOLUTIONS 
404 

cover  in  the  floor  of  the  fossa  completely,  the  groove  which  nmrks 
thlline  of  their  contact  forming  the  lateral  cerebral  fissure,  wh.le  the 
floor  of  the  fossa  becomes  known  as  the  mi«/o. 

The  first  of  the  secondary  fissures  to  appear  is  the  sulcus  c,n^^ 
which  is  formed  about  the  middle  of  the  fifth  month  on  the  me^.a 
Turface  of  the  hemispheres,  lying  parallel  to  the  anterior  port  on  o 
he  hippocampal  fissure  and  dividing  the  mes.al  ="rf-<^<=.  •"!°;'^^ 
,wiS«afc  .nAforr^icalus.    A  little  later,  at  the  beg,nnmg  of 
fhTsixth  month,  several  other  fissures  make  their  appearance  upon 


the  outer  surface  of  the  pallium,  the  chief  of  these  being  the  cerUral 
'ITinrU^-parieUU^.  pre-  and  Posl-^^rai^^^^^^;^--^^^^^ 
sulci  the  most  ventral  of  these  last  runnmg  parallel  w.th  the  lower 
;S;on  of  the  hippocampal  fissure  and  differing  from  the  0  Jrs  u, 
forming  a  ridge  on  the  wall  of  the  ventricle  termed  the  coUat^al 
™e,  whence  the  fissure  is  known  as  the  colUUeral.  The  posmon 
oTmost  ;f  these  fissures  may  be  seen  from  Fig.  .43,  and  for  a  more 


^"V^ 


THE    CORPUS   CALLOSDM 


405 


complete  description  of  them  reference  may  be  liad  to  text-books  of 
descriptive  anatomy. 

In  later  stages  numerous  tertiary  fissures  make  their  appearance 
and  mask  more  or  less  extensively  the  secondaries,  than  which  they 
are,  as  a  rule,  much  more  inconstant  in  position  and  shallower. 
The  Corpus  Callosum  and  Fornix.— VihWe  these  fissures  have  been 
forming,  important  structures  have  developed  in  connection  with 
the  lamina  terminalis.  Up  to  about  the  fourth  month  the  lamina 
is  thin  and  of  nearly  uniform  thickness  throughout,  but  at  this  tim ; 
it  begins  to  thicken  near  its  dorsal  edge  and  fibers  appear  in  the 
thickening.  These  fibers  belong  to  three  sets.  In  the  first  place, 
certain  of  them  arise  in  connection  with  the  olfactory  tracts  (see  p. 
407)  and  from  the  region  of  the  hippocampal  gyrus,  which  is  also 
associated  with  the  olfactory  sense,  and,  passing  through  the  sub- 
stance of  the  lamina  terminalis,  they  extend  across  the  median  line 
to  the  corresponding  regions  of  the  opposite  cerebral  hemisphere. 
They  are  therefore  commissural  fibers  and  form  what  is  termed  the 
anterior  commissure  (Figs.  244,  ca  and  245,  ac).  Secondly,  fibers, 
which  have  their  origin  from  the  cells  of  the  hippocampus,  develop 
along  the  chorioidal  edge  of  that  structure,  forming  what  is  termed 
Ihe  fimbria.  They  follow  along  the  edge  of  the  chorioidal  fissure 
and,  when  this  reaches  the  interventricular  foramen,  they  enter  as 
the  pillars  of  the  fornix  (Figs.  244,  cf;  Fig.  245,/)  the  substance  of  the 
lamina  terminalis  and,  passing  ventrally  in  it,  eventually  reach  the 
hypothalamic  region,  where  they  terminate  in  the  corpora 
mammillaria. 

Thirdly,  as  the  mantle  develops  fibers  radiate  from  all  parts  of 
it  toward  the  dorsal  portion  of  the  lamina  terminalis  and  traversing 
it  are  distributed  to  the  corresponding  portions  of  the  mantle  of  the 
opposite  side.  There  fibers  are  also  commissural  in  character  and 
form  the  corpus  callosum  (Figs.  244  and  245,  cc).  With  the  develop- 
ment of  these  three  sets  of  fibers  and  especially  those  forming  the 
corpus  callosum,  the  dorsal  portion  of  the  lamina  terminalis  be- 
comes enlarged  so  as  to  form  a  triangular  area  extending  between 
the  two  cerebral  hemispheres  (Fig.  243),  the  corpus  callosum  form- 


4o6 


THE    COKFUS  CAIXOSmt 


ing  its  dorsal  portion  and  base,  which  is  directed  anteriorly,  the 
pillars  of  the  fornix  its  ventral  portion,  while  the  anterior  commissure 
occupies  ite  ventral  anterior  angle. 

The  portion  of  the  triangle  included  between  the  callosum  and 
the  fornix  remains  thin  and  forms  the  septum  peUucidum,  and  a  split 
occurring  in  the  center  of  this  gives  rise  to  the  so-called,</i*  ventricle, 

which,  from  its  mode  of  forma- 
tion, is  a  completely  closed  cav- 
ity and  is  not  lined  with  epen- 
dyma!  tissue  of  the  same  nature 
as  that  found  in  the  other  ven- 
I  tricles. 

Owing  to  the  very  consider- 
able size  reached  by  the  trian- 
■  gular  area  whose  history  has  just 
been  described,  important 
changes  are  wrought  in  the  ad- 
joining portions  of  the  mesial 
surface  of  the  hemispheres.  Be- 
fore the  development  of  the  area 

„.,  c„  the  gyrus  dentatus  and  the  hip- 

Fio  244. — MiDiAN  LONorroDiNAL  Sec-  •■">•  6J  ' 

HON  o»  TBI  BnAiH  or  AN  EiiBiYo  OF  pocampus  extcud  forward  mto 

Fora  MONTHS.  ^   anterior  porUon  of  the  hem- 

c  Cdlcaiine  fissure;  ca,  anteno    com-     **  r 

miMure;  a,  corpus  caUosum;c/.  chcnoidal    ispheres    (Fig.    244),  but  on  ac- 
count of  their  position  they  be- 
come encroached  upon  by  the 
enlargement  of  the  corpus  callo- 
sum, with  the  result  that  the  hippocampus  becomes    practically 
obliterated  in  that  portion  of  its  course  which  lies  in  the  region 
occupied  by  the  corpus  callosum,  its  fissure  in  this  region  becoming 
known  as  the  callosal  fissure,  while  the  corresponding  portions 
of  the  dentate  gyrus  become  reduced  to  narrow  and  insignificant 
bands  of  nerve  tissue  which  rest  upon  the  upper  surface  of  the  corpus 
callosum  and  are  known  as  the  lateral  longitudinal  stria. 

The  Olfactory  Lobes.— Kt  the  time  when  the  cerebral  hemispheres 


fissure;  dg,  denute  gyrus;  Jm,  ir.terven 
tricular  foramsn;  »,  hippocampal  fissure: 
to,  pariettMJCcipital   fissure.— (Mtioi- 


THE   OLFACTOKY  LOBES 


407 


begin  to  enlarge-that  It  to  say,  at  about  the  fourth  week-a  slight 

fuiTow,  which  appears  on  the  ventral  surface  of  each  anteriorly 

marks  off  an  area  which,  continuing  to  enlarge  with  the  hemispheres,' 

gradually  becomes  constricted  off  from  them  to  form  a  distinct  lobe- 

hke  structure,  the  olfactory  lobe  (Fig.  233,  VI  3).    In  most  of  the 

lower  mammalia  these  lobes 

reach  a  very  considerable  size,  ri 

and  consequently  have  been  ^.^-'-4^»^     <*« 

regarded   as  constituting  an 

addition^  division  of  the 

brain,  known  as  the  rhinen- 

cephalon,   but   in   man   they  cc 

remain  smaller,  and  although      , 

they  are  at  first  hollow,  con-    "^ 

taining  prolongations  from  the 

lateral  ventricles,  the  cavities 

later  on  disappear  and  the 

lobes    become    solid.    Each 

lobe    becomes    differentiated 

into  two  portions,  its  terminal 


Fio.  345.— Median  LoNomTDmAL  SicnoN 

OF  THE  B«AIN  OF  AN  ElIMVO  OF  THE  FlITH 

Month. 

ac,  Anterior  commissuTe;  cc,  corpus  callo- 

sum;  dg,  dentate  gyrus;/,  fomii;  i,  infundib- 

..         ,  .   '  "    ulum;    Hie    intermediate    mass:    si    sentum 

portion  becoming  converted   peiiucidum;»,-,,eluminterpo.itim-(S51 

into  the   club-shaped   stnic-  > 

ture,  the  olfactory  bulb  and  stalk,  while  its  proximal  portion  gives 

rise  to  the  olfactory  tracts,  the  trigone,  and  the  anterior  perforated 

substance. 

Histogenesis  of  the  CerebrcU  Cortex.— \  satisfactory  study  of  the 
histogenesis  of  the  cortex  has  not  yet  besn  made.  In  embryos  of 
three  months  a  marginal  velum  is  present  and  probably  gives  rise 
to  the  stratum  zonale  of  the  adult  brain;  beneath  this  is  a  cellular 
ayer,  perhaps  representing  the  mantle  layer;  beneath  this,  again,  a 
layer  of  nerve-fibers  is  beginning  to  appear,  represeming  the  white 
suostance  of  the  pallium;  and,  finally,  lining  the  ventricle  is  an 
cpendym:.!  layer.  In  embryos  of  the  fifth  month,  toward  the  in- 
nermost part  of  the  second  layer,  cells  arc  beginning  to  differentiate 
into  the  large  pyramidal  cells,  but  almost  nothing  is  known  as  to  the 


4o8 


THE    SPINAL  NERVES 


origin  of  the  other  layers  recognizable  in  the  adult  cortex,  nor  is  it 
known  whether  any  migration,  similar  to  what  occurs  in  the  cere- 
bellar cortex,  takes  place.  The  fibers  of  the  white  substance  do  not 
begin  to  acquire  their  myelin  sheaths  until  toward  the  end  of  the 
ninth  month,  and  the  process  is  not  completed  until  some  time  after 
birth  (Flechsig),  while  the  fibers  of  the  cortex  continue  to  undergo 
myelination  until  comparatively  late  in  life  (Kaes). 

The  Development  of  the  Spinal  Nerves. — It  has  already  been 
seen  that  there  is  a  fundamental  difference  in  the  mode  of  develop- 
ment of  the  two  roots  of  which  the  typical  spinal  nerves  are  composed, 
the  ventral  root  being  formed  by  axis-cylinders  which  arise  from 
neuroblasts  situated  within  the  substance  of  the  spinal  cord,  while 
the  dorsal  roots  arise  from  the  cells  of  the  neural  crests,  their  axis- 
cylinders  growing  into  the  substance  of  the  cord  while  their  dendrites 
become  prolonged  peripherally  to  form  the  sensory  fibers  of  the 
nerves.  Throughout  the  thoracic,  lumbar  and  sacral  regions  of  the 
cord  the  fibers  which  grow  out  from  the  anterior  horn  cells  converge 
to  form  a  single  nerve-root  in  each  segment,  but  in  the  cervical  region 
fibers  which  arise  from  the  more  laterally  situated  neuroblasts  make 
their  exit  from  the  cord  independently  of  the  more  ventral  neuro- 
blasts and  form  the  roots  of  the  spinal  accessory  nerve  (see  p.  416). 
In  the  cervical  region  there  arc  accordingly  three  sets  of  nerve-roots, 
the  dorsal,  lateral,  and  ventral  sets. 

In  a  typical  spinal  nerve,  such  as  one  of  the  thoracic  series,  the 
dorsal  roots  as  they  grow  peripherally  pass  ventrally  as  well  as  out- 
ward, so  that  they  quickly  come  into  contact  with  the  ventral  roots 
with  whose  fibers  they  mingle,  and  the  mixed  nerve  so  formed  soon 
after  divides  into  two  trunks,  a  dorsal  one,  which  is  distributed  to  the 
dorsal  musculature  and  integument,  and  a  larger  ventral  one.  The 
ventral  division  as  it  continues  its  outward  growth  soon  reaches  the 
dorsal  angle  of  the  pleuro-peritoneal  cavity,  where  it  divides,  one 
branch  passing  into  the  tissue  of  the  body- wall  while  the  other  passes 
into  the  splanchnic  mesoderm.  The  former  branch,  continuing  its 
onward  course  in  the  body-wall,  again  divides,  one  branch  becoming 
the  lateral  cutaneous  nerve,  while  the  othw  continues  inward  to 


THE    CRANlAl  NEBVES 


409 


terminate  in  the  median  ventral  portion  of  the  body  as  the  anterior 

aTt^  the"""'-    .1"^  ^'"""^""'^  •'''*-'> '--  ^  ramVs   olr 
cans  to  the  sympathet.c  system  and  will  be  considered  more  Jy 

.helrtrpart  "Stf '"'''  ''"  ''"'^  ^^■'^^'^  °'"»'"  '»-°««''OUt 
c  greater  part  of  the  thoracic  reg  on.     Elsewhere  thp  fiK»™Tf  .1, 

ventral  divisions  of  the  nerves  as  they  grow  ^^l^  tend  to  pa,  I 
from  one  another  and  to  become  associated  with  the  fibers  of  adt 
ZIT7  "T  "1 1°  '''^^""'-     ^"  '"^  -«--  where  he  £ 

m  connecfon  with  the  idea  that  an  accurate  knowledge TS 
development  would  afford  a  clue  to  a  most  vexed  prob  em  of  v  r^ 
brate  morphology,  the  metamerism  of  the  head.     That  the  met 
mens„  ,h,h  ,,,      ^^^^^^^^^^  .^  ^^^  ^^^^^  extend iro  the 

head  was  a  natural  supposition,  strengthened  by  the  discovery  ol 
head-cavities  in  the  lower  vertebrates  and  by  the  ind  caZs  o 
metamensm  seen  in  the  branchial  arches,  and  the  proWem  which 

hev  had  utr"  ".  "'  determination  of  the  modifications  ZI 
they  had  undergone  durmg  the  evolution  of  the  head 

In  the  trunk  region  a  nerve  forms  a  conspicuous  element  of  each 
metamere  and  .s  composed,  according  to  what  is  known  as  Bel  ' 
law,  of  a  ventral  or  efferent  and  a  dorsal  or  afferent  root      Unti 
comparatively  recently  the  study  of  the  cranial  nerves  Is  been 

o  Bell  s  law  to  them  and  to  recognize  in  the  cranial  region  a  number 

location,  as  it  were,  01  the  two  roots  had  occurred 

The  results  obtained  from  investigation  along  this  line  have  not 


410 


THE  OANIAL  MZ>VX8 


however,  proved  entirely  satisfactory,  and  facts  have  been  elucidated 
which  seem  to  show  that  it  is  not  possible  to  extend  Bell's  law,  in  its 
usual  form  at  least,  to  the  cranial  nerves.  It  has  been  found  that 
it  is  not  sufficient  to  recognize  simply  afferent  and  efferent  roots, 
but  these  must  be  analyzed  into  further  components,  and  when  this 
is  done  it  is  found  that  in  the  series  of  cranial  nerves  certain  com- 
ponents occur  which  are  not  represented  in  the  nerves  of  the  spinal 
series. 

Before  proceeding  to  a  description  of  these  components  it  will  be 
well  to  call  attention  to  a  matter  already  alluded  to  in  a  previous 
chapter  (p.  84)  in  connection  with  the  segpientation  of  the  meso- 
derm of  the  head.  It  has  been  pointed  out  that  while  there  exist 
"head-cavities"  which  are  serially  homologous  with  the  mesodermal 
somites  of  the  trunk,  there  has  been  imposed  upon  this  primary 
cranial  metamerism  a  secon,dary  metamerism  represented  by 
the  branchiomeres  associated  with  the  branchial  arches,  and, 
it  may  be  added,  this  secondary  metamerism  has  become  the  more 
prominent  of  the  two,  the  primary  one,  as  it  developed,  gradually 
slipping  into  the  background  until,  in  the  higher  vertebrates,  it  has 
become  to  a  very  considerable  extent  rudimentary.  In  accordance 
with  this  double  metamerism  it  is  necessary  to  recognize  two  sets  of 
cranial  muscles,  one  derived  from  the  cranial  myotomes  and  repre- 
sented by  the  muscles  of  the  eyeball,  and  one  derived  from  the 
branchiomeric  mesoderm,  and  it  is  necessary  also  to  recognize 
for  these  two  sets  of  muscles  two  sets  of  motor  nerves,  so 
that,  with  the  dorsal  or  sensory  nerve-roots,  there  are  altogether 
three  sets  of  nerve-roots  in  the  cranial  region  instead  of  only  two,  as 
in  the  spinal  region. 

These  three  sets  of  roots  are  readily  recognizable  both  in  the  em- 
bryonic and  in  the  adult  brain,  especially  if  attention  be  directed  to 
the  cell  groups  or  nuclei  with  which  they  are  associated  (Fig.  24O,. 
Thus  there  can  be  recognized:  (i)  a  series  of  nuclei  from  which 
nerve-fibers  arise,  situated  in  the  floor  of  the  fourth  ventricle  and 
aqujeduct  close  to  the  median  line  and  termed  the  vmtrai  motor 
nuclei;  (2)  a  second  series  of  nuclei  of  origin,  situated  more  laterally 


rat  C«AMML  MUVES  ^,, 

«^  te  the  »ubsta|,ce  of  the  fonnatio  reticularis,  and  known  as  .he 
^^'-"or  »«./«;  and  (3)  a  series  of  nuclei  in  which  affena,,  nerve- 

^^arne^r"*  *,^'  '^"'  "'  '"""^  »'""•■     No-  °f  the  twelve 

Sailed  wr^l"'  "'Tu'"'  '"  ''''  *"'-^'"  ^"'^-^  fibe" 
abated  w>th  all  three  of  these  nuclei;  the  fibers  from  the  lateral 

motor  nucle.  almost  invariably  unite  ,  ith  sensory  fibers  to  form  a 


GLOSSAL  (Xll)  NE»VIS.-(H«.)  ""^  °'  ™^  ^AOUS  (JQ  AKD  Hypo- 

^^S"^^:  ^"V-^'t^™""  *"  "'^  ^^°''^'  '"oto^  »"clei  form  inde- 
pendent roots,  while  the  olfactory  and  auditory  nerves  alonrnr^ll 

omafnr  T  ^°!t"'"«  ^"^  '•■^  P^--'  thZoprne^  )Xo 
contam  fibers  from  either  of  the  series  of  motornuclei.    The  relations 
of  the  various  cranial  nerves  to  the  nuclei  may  be  seen  frnr?^ 
following  table,  in  which  the  +  sign  indicateHL  presZrd  he 
-  s^n  the  absence  of  fibers  from  the  nuclear  series  unde^^^h': 


412 


THE   CSANIAL   NERVES 


Nttmbn 

Nunc 
Olfactory. 

Vcnin'  Motor 

LilenI  Motor 

Seuoiy 

I. 

"T 

"    '_ 

+  ' 

III. 

Oculomotor. 

+ 

- 

- 

IV. 

Trochlnr. 

+ 

- 

- 

V. 

Trigeminus. 

- 

+ 

+ 

VI 

Abducens. 

+ 

— 

— 

VII.    • 

Facial. 

+ 

+ 

VIII. 

Auditory. 

- 

- 

+ 

IX. 

Ulosisopliaryngeal. 

- 

+ 

+ 

X. 
XI. 

Vagus. 

Spinal  Accessory. 

~ 

+ 

+ 

Two  nerves — namely,  the  second  and  twelfth — have  been  omitted 
from  the  above  table.  Of  these,  the  sorond  or  optic  nerve  undoubt- 
edly belongs  to  an  entirely  different  cp-ii  yory  from  the  other  periph- 
eral nerves,  and  will  be  considered  in  the  following  chapter  in 
connection  with  the  sense-organ  with  which  it  is  associated  (see 
especially  p.  460).  The  twelfth  or  hypoglossal  nerve,  on  the  other 
hand,  really  belongs  to  the  spinal  series  and  has  only  secondarily 
been  taken  up  into  the  cranial  region  in  the  higher  vertebrates.  It 
has  already  been  seen  (p.  170)  that  the  bodies  of  four  vertebrae  are 
included  in  the  basioccipital  bone,  and  that  three  of  the  nervis 
corresponding  to  these  vertebrae  are  represented  in  the  adult  by  the 
hypoglossal  and  the  fourth  by  the  first  cervical  or  suboccipital  nervi-. 
The  dorsal  roots  of  the  hypoglossal  nerves  seem  to  h,.ve  almost 
disappeared,  although  a  ganglion  has  been  observed  in  embryos  of 
7  and  10  mm.  in  the  posterior  part  of  the  hypoglossal  region  (His), 
and  probably  represents  the  dorsal  root  of  the  most  posterior  portion 
of  the  hypoglossal  nerve.  This  ganglion  disappears,  as  a  rule,  in 
later  stages,  and  it  is  interesting  to  note  that  the  ganglion  of  the 
suboccipital  nerve  is  also  occasionally  wanting  in  the  adult  condition. 
The  hypoglossal  roots  are  to  be  regarded,  then,  as  equivalent  to  the 
ventral  roots  of  the  cervical  spinal  nerves,  and  the  nuclei  ln<ta 
which  they  arise  lie  in  series  with  the  cranial  ventral  motor  roots,  a 


™e  cranml  nerves 
cord.  "'"""  °f  "><=  ""'enor  horns  of  the  spinal 

the  fibers  which  are  di  tnbu  edt^  ,f.  ^  ,'^  °"'  ^'"^  """  """o  «'« 
In  .he  case  of  the  s^^r;  nerve ':  S"  °' ''?'' '""^'■'°'""- 
before  their  equivalents  Z  Z  ,         '  """'j"''  '"'  "^^^^ary 

For  this  the  studies  whchhiv  k"'""  ''"''  ''"'  ^  determineZ 
components enteringithecrniaf"  "'''  '"  """'  ^^^  "^  'he 
and  fishes  (Herrick)Vul°  ^ZlTu  "• "''  "'  ""^  "'"P'''^'''  (Strong) 
ofthe  mammalian  nervha,lf'^"^'='r'''y^'-''irec.analyS 

■•'  has  been  found  Tt  ^te  di£e"  "  '  '"  "'  '"'"'  "'""^'^ 
formation  of  the  dorsal  root  of  th!,'^"'"''  ""'"  '"">  'he 
■•ng  to  a  general  cutan  Z "i™  "^"'^'^  ^  fibers  belong, 
the  skin  without  being  connected^  hlT"''  '^?'"'  '"^'"^"'^'^  "> 
fibers  belonging  to  what  is  te^  '  Zr "'  ^'^"--«-^;  (^) 
^y^/«w,  distributed  to  the  wallHf  thl  '""V"""!^  '»•  ^•"^''<'-^«..»ry 
and  to  special  organs  fould  ;  he  ^kT  fh  ''"''T'  '^«"°" 
those  occurring  in  the  mouth-  ^nTrTl  '^""^  character  as 

set  of  cutaneous  senseZan^  T     '^  ^^  *"='''"«'"«  '"  ^  ^Pecial 
known  as  theorgarofZrateSt'^^""''^''  '"  '"^  '^^"^  -"^ 

ce"s.  sftua^ed  i^t  ,:r;;r«THrr  7r^ '« ^  ^-^  - 

and  forming  what  is  termed  tt,  °^  "*'  '°"«''  ^'^^"'f'^''^ 

posteriorly  in  the  su4ta™e  of  tt'^.T"''  '°'"='  "'"'  ^'^o  ^^'-d 
has  been  termed  the  spTnl        f  ofTht^  ^^'^^  '''^'  '°™'''«  ^""^^ 

posterior  horn  of  the  cord  r  ",.  Vt'^'^^ntmuationofthe 
belonging  to  this  system  are  to  L  f  !  '  """^  ^'"P'^'''^  fibers 
tenth  nerves,  but  in  he  mamm.1  Z^"!'"  '"'  '^'"''  ^^^^"'h,  and 
become  more  limitJdTeirci^  '^T  "f "''""°" ''''«  "PP^^^-'x 
trigeminus,  of  whoslLondi  1'"'"°^'  ^"^'"^'^^'y  '»  the 

part.  Since  the  cZ  r2  wSTeS  'T^  ^''^  ^""^''^-'"^ 
trigeminus  terminate  are  the  7orw»  H  "'  ""=  'P'""'  ^°«  ^^ "»« 

the  forward  continuations  of  the  posterior 


414 


TBX  CBAMUL  inKVKt 


horn  of  the  cord,  it  seem,  probable  that  the  fibers  of  thi.  system 
are  the  cranUl  representatives  of  the  posterior  roots  of  the  spinal 
nerves,  which,  it  lay  be  noted,  are  also  somatic  in  their  distribution. 
The  fibers  of  the  viscerosensory  system  are  found  in  the  lower 
forms  principaUy  in  the  ninth  and  tenth  nerves  (see  Fig.  a47). 
although  groups  of  them  are  also  incorporated  in  the  seventh  and 
fifth.  They  converge  to  a  mass  of  cells,  known  as  the  lobus  vagi, 
and  like  the  first  set  are  also  continued  down  the  medulla  to  form 


nx 


FtO    «7  — DlAOIAlB  SHOWDiO  THE  SiNSORY  CoHPOOTNTS  0»  THE  C»AMML  NE»VTS 

"'  ofaFish  (MwtWw)- 

The  5om»tic  MMory  system  U  unshaded,  the  viscero«iBOryU  cross-hatah^,  an.i 
the  Uter"toe  system  b\l£k.    a,c.v.  Spinal  root  of  trigeonnus;  h,x.  ^IJf^^?^^. 
of  virus  te  lobus  vagi;  oi.  olfactory  bulb;  of,  optic  nerve;  rc.»,  ^t*"""" ^*?^'' °|  ""^ 
vagT^faT  Sm5*braich  of  v^us;  W,  Wal  line  "'"=;  J  "^."S"^ J^  , 
ItaTe  n^;  f «,  superficial  ophthalmic;  rf,  ramus  palaum^of  the  fac«l;  rty,  hyom«>d.l 
ular  branch  of  the  facUl;  l.<«/,  mfraorbital  nerve.— (Hbtk:*.) 

a  tract  known  as  X\^fas(Aaiim  solUarius  ot  fasciculus  communis.  In 
the  mammaUa  the  system  is  represented  by  the  sensory  fibers  of  the 
glosso-pharyngeo-vagus  set  of  nerves,  of  which  it  represents  pra - 
tically  the  entire  mass;  by  the  sensory  fibers  ot  the  facial  arising  from 
the  geniculate  ganglion  and  included  in  the  chorda  tympam  and 
probably  also  the  great  superficial  petrosal;  and  also,  probably,  \>y 


IHt   OANUL  NEKVBI 


4»S 


tme  branch  from  the  facial,  i,  «.em,  probable  that  the  palatine  ne^cs 
t   hi  T  *''. "''"  '°  ^  '""■«"'d  '0  'h»  system.*    If  Z 

swtem  Tn^;'  '""  7'^""'  '''"'  "  "^"^'l'^''  '»  '"e  homologies  of  the 
rtol  r.lJ!!"'"  "*'^'''  '°'  ''■""  '"'  'Pheno-palatine  ganglion 

butS S'tlllf  t1  '°  ""=  ^^^''^  °'  ""^  ""'^""  "-•  '"ere  seems 
nerve       T.  i  ^^  ""  '^P^^^ntation  whatsoever  in  the  spinal 

oZs  found  oT"''"  "'■'  "  P^'"""  ^^"^'"  of  cutaneous  se'nse- 
Xnrv  H  ^u?  '"'"'"''  °'  ""■■"<>  ■""■">«'«-  and  also  with  the 
aud^ory  and  poss.bly  the  olfactory  organs,  the  former  of  wh   h  Ire 

So2h  th^    ■         '  •"«'"''  ■"*  P"°"'P^"y  '^<">fi"<^d  to  the  head, 

ya  b^nchlmTh"'"'  "^"  '''= '™"'''  "''"^  ''"^^  "'  '"""-d 
suddhTk  '"'" '^''^^K""  "erve,  the  entire  system  being  accordingly 

""helS;,""''/  "'"":  '"  "^"^  ^'^''^'  '■"  -•>-»•  the  developS 
found^3;'^^' -^  r '""'"■  '•""  '"'°''«'°«  '»  '^e  systeL  are 
of  ,h.  Z  7,  '"'^'"^"ch.omeric  nerves  and  all  convenje  to  a  portion 

mat  whh  tt^°""  "  '"^  "^""'•""  "-'-«•    I"  the  Mam 
b^n  ;  d  It.        '■'1'"!"''  °'  '""^  '*"=^'  ""«  °^«*»«  there  has 
repre^ntauve  of  the  system  which  persists  is  the  auditory  nerve. 
thn„ah  .        *?'"'°  °°  P^«'= '»'-  "^y  ""'^  ^  expanded  as  follows 

T^eLTr /.  "°"  ^'■°'"  '''"'  "^^  ^'^  ''^^"^  '■"  lower 

nervT^  ^  °'*°*'^  '"■"  *«  ""P"'""'  gangUon  to  ^„  into  dth« 


41 6 


THE   CRANIAL  NERVES 


forms,  and  may  require  some  modifications  when  the  components 
have  been  subjected  to  actual  observation: 


Nerve 

Venlnil 
Motor 

LatenI 
Motor 

I 

_ 

_ 

III. 

+ 

- 

IV. 

+ 

- 

V. 

- 

+ 

VI. 

+ 

- 

VII. 

- 

+ 

VIII. 

- 

- 

IX. 

X. 

- 

+ 

XI. 

XII. 

+ 

- 

Spinal. 

+ 

(?) 

Somatic 
Sensory 


Visceral 
Sensory 


Lateral 

Line 


An  additional  word  is  necessary  concerning  the  spinal  accessory 
nerve,  for  it  presents  certain  interesting  relations  which  possibly 
furnish  a  clue  to  the  spinal  equivalents  of  the  lateral  motor  roots. 
In  the  first  place,  the  neuroblasts  which  give  rise  to  those  fibers  of 
the  nerve  which  come  from  the  spinal  cord  are  situated  in  the  dorsal 
part  of  the  ventral  zones.  As  the  nuclei  of  origin  are  traced  anter- 
iorly they  will  be  found  to  change  their  position  somewhat  as  the 
medulla  is  reached  and  eventually  come  to  lie  in  the  reticular  forma- 
tion, the  most  anterior  of  them  being  practically  continuous  with 
the  motor  nucleus  of  the  vagus.  Indeed,  it  seems  that  the  spinal 
accessory  nerve  is  properly  to  be  regarded  as  an  extension  of  the 
vagus  downward  into  the  cervical  region  (Furbringer,  Streeter), 
a  process  which  reaches  its  greatest  development  in  the  mammalia 
and  seems  to  stand  in  relation  to  the  development  of  those  portions 
of  the  trapezius  and  stemo-mastoid  muscles  whirh  are  supplied  by 
the  spinal  accessory  nerve. 

It  is  believed  that  the  white  rami  communicantes  which  pass 
from  the  spinal  cord  to  the  thoracic  and  upper  lumbar  sympathetic 


THE   CRANUL  NERVES 

venSl  hotV"""'".  -r"'  "■""'"='  '■"  ''^  ''"""-'-'e^'  Portions  of  the 
ventral  horns,  and  .t  .5  noteworthy  that  white  rami  are  wantinir  in 
the  reg.o„  .„  which  the  spinal  accessory  nerve  occu^  "n  e  fh  " 
nenre  represents  a  cranial  lateral  motor  root  the  temptation  is  Jea 
to  regard  the  cranial  lateral  motor  roots  as  equivalent  toThl  f^f 
«m.  of  the  cord  and  this  temptation  is  in.ensifirw  i  tl  ,2 

^^ritrthTbSv"'^"'"*^'^''' ''"''  '"p-^^^p-^-'  —  - 

r^;erSirsr;s::r^irr~ 
=rs£ScS----«-~ 

awa^t  further  studies  before  yielding  to  theTemptX  ^  '° 

As  regards  the  actual  development  of  the  cranial' nerves   thev 

ml:  fit  'T"'  •*"  "''■■^''  "'"*'-  f-  '"^  spinal  ne^;    the 
motor  fibers  bemg  outgrowths  from  neuroblasts  situat^  in  the 
waUs  of  the  neural  tube,  while  the  sensory  nerves  are  o^ow  1 
from  the  cells  o  ganglia  situated  without  the  tube.    In  thTlowe 
v^ebrates  a  senes  of  thickenings,  known  as  the  suprabranchiJ2 

S  'the'levrS  'T-;"=  ""^'™  "'""^  ^  "ne  corres^ndli 
wuh  the  level  of  the  auditory  mvagination,  while  on  a  linrcorre 
spondmg  w,th  the  upper  extremities  of  the  branchial  cLts  anothe 

and  with  both  of  these  sets  of  placodes  the  cranial  nerves  arc  in 

been  found  m  connection  with  t„e  fifth,  seventh,  ninth  and  tenth 
n  rves,  to  whose  ganglia  they  contribute  cells.  The  suprabranchial 
placodes,  which  in  the  lower  vertebrates  are  associated  whh  the 
lateral  hne  nerves,  are  unrepresented  in  man,  unless,  as  has  been 
maintained  the  sense-organs  of  the  internal  ear  are  the" 
representatives.  '"^"^ 


4i8 


THE  SYMPATHETIC  SYSTEM 


III 


With  one  another  nor  «jth  U.e  ^cal  sprn^  nerve^^^^^^^^^^ 

regarded  as  represenung  twelve  aama^se^mente^n^       ^  ^^ 

&lt  comparatively  l''^.  ^°™<1  to^lr Tform  the  head  is  to  be 
myotomic  segments  which  have  "?«<'*?il,  there  are  only  four  of  these 
derived  from  the  crama^  ^A  .fricm«seq^val"nt  tfflie  mesodermic 
nerves  which  are  assoaated  "'* ''^ '^"'^Tof  head  cavities  or  meso- 
somites  of  the  trunk  a  much  greater  """"7' ?J^^?„  „{  the  embryos 
dermic  somites  has  been  obsenr.^  m  ^c  amal  r^^^^^^    ^^^^^^  l^^ 

or  whether,  indeed,  any/°"'='P°°'l;""J^i'''or^'°ttictions  have  been 

In  early  stages  ?«  f^^P^neSubeTudhavTbeen  regarded 

observed  in  the  cramal  P"''°"°'X"*fXt  structure.     The  neuromeres, 

as  indicating  a  primiUve  segmentet'on  of  tot  st^^^^  ^^^ 

sponding  to  the  various  nerves. 

The  Development  of  the  Sympathetic  Nervous  System.- 
Fr»rthe  embry^cal  standpoint  the  distinction  wh,ch  has  been 

:^rwtJ:^ori: --  --ode  o.  ^^.n  has^^enob.^. 
with  especial  clearness  in  the  embryos  of  some  of  the  lower  verte 
Ste  rich  masses  o£  cells  have  been  seen  to  separate  ^m  the 
^sferior  It  ganglia  to  form  the  ganglia  of  the  ganghonated  cord 
KTs)  In'^h!  mammalia,  including  man,  the  relations  oth 
two  sete  of  ganglia  to  one  another  is  by  no  means  so  apparent,  smcc 
I  s'mitCtic  cells,  instead  of  being  sepa^ed  from  the  posuno 
Jot  gan^ion  en  masse,  migrate  from  it  singly  or  m  groups,  and  arc 
Iwore  less  readily  distinguishable  from  the  surroundmg  meso- 

'To  Sl'tand  the  development  of  the  sympathetic  system  it 
musTbe  «membered  that  it  consists  typically  of  three  sets  of  gar. 
r  OnZ  these  is  constituted  by  the  ganglia  of  the  g^^ng^f 
corf  (Fig.  H9.  GC),  the  second  is  represented  by  the  ganglia  of  the 


THE   SYItPATBETIC  SVSTElf 


1'  ^1 


419 


FeO.  J48._T«ANSVI«S£  SECnON  THROUGH  AN  EltBKYO  ShAIK  (ScyUium)  0»  IS  MM 
SHOWMO  TBI  OUOIN  OF  A  SrMPATHBTIC  QaNOUON. 

C*.  Nolochord;  £,  ectoderm;  '^•^^'^^^g^^^''  <^'.  sympatheUc  ganglion;  U, 


■  JO  THE    SYMPATHETIC   SYSTEM 

pravertebral  plexuses  (PVG),  such  as  the  cardiac,  solar,  hyP<«a*- 
tric,  and  pelvic,  while  the  third  or  peripheral  set  (PG)  is  formed  by 
the  cells  which  occur  throughout  the  tissues  of  probably  most  of  the 
visceral  organs,  either  in  small  groups  or  scattered  through  plexuses 
such  as  the  Auerbach  and  Meissner  plexuses  of  the  intestme.  Each 
cell  in  these  various  ganglia  stands  in  direct  contact  with  the  axis- 
cylinder  of  a  cell  situated  in  the  central  nervous  system,  probably  m 
the  lateral  horn  of  the  spinal  coid  or  the  corresponding  region  of  the 
brain,  so  that  each  cell  forms  the  terminal  link  of  a  chain  whose  first 
link  is  a  neurone  belonging  to  the  central  system  (Huber).   Through- 


FlO.  940  -DttORAK  SHOwraO  THE  AMANGEMENI  OF  THE  NeOTONES  OF  IHE  S™pA- 

'*  THiTic  System. 

The  6bers  from  the  posterior  root  ganglia  are  represented  by  the  broken  "ack  Uhk; 
thoifrom  Se^^or  horn  cells  by  the  solid  black;  the  wh.'te  ram,  by  «d;  and  the 
sl^aSSc  neurones  by  blue.  M,  Do.«l  ramus  of  spinal  n«ye  ;GC,  ^^'^ 
J«rX  GR  mv  ramus  commu.iicans;  PC,  peripheral  ganglion;  FVC,  prevertebral 
^u™;  Vfi^veS  ramus  of  .pinal  ne-Te;  WR.  »hite  ramus  commumcans.- 
(Adapled/rom  Huber.) 

out  the  thoracic  and  upper  lumbar  regions  of  the  body  the  central 
system  neurones  form  distinct  cords  known  as  the  whUe  ramt  com- 
municanles  (Fig.  249,  WR),  which  pass  from  the  spinal  nerves  to  the 
adjacent  ganglia  of  the  ganglionated  cord,  some  of  them  terminal 
ing  around  the  cells  of  these  ganglia,  others  passing  on  to  the  cells .. 
the  prsBVertebral  ganglia,  and  others  to  those  of  the  peripheral 
plexuses  In  the  cervical,  lower  lumbar  and  sacral  regions  white 
rami  are  wanting,  the  central  neurones  in  the  first-named  region 
probably  making  their  way  to  the  sympathetic  cells  by  way  of  the  upper 


THE   SYMPATHETIC  SVSTEM 

421 

^nT"  TiT"' J"!""  '"  '"'  ^"""^  "«'°°=  'hey  may  pass  down  the 
gangl.onated  cord  from  higher  regions  or  may  join  thrpr^vertebra 
and  peripheral  ganglia  directly  without  passing  through  the  M 

fll~?u  "''"  '""'""^  '«='^*^"  'he  proximal  ganglia  and  the  spinal 
nerves;  these  are  composed  of  fibers,  arising  from  sympathetic  cells 


Fio.  250.— Tmnsvirse  Section  thmugh  thi  Spto^i  r„,„  „. 

iiiS^HsStr '  ™  '"  ""'"  '°  ^^^  '''"^  "^^^  -  'heir 
The  brief  description  here  given  applies  especially  to  the  sym- 
pathetic system  of  the  neck  and  trunk.  Represen',  .,  of'^Te 
system  aie  also  found  in  the  head,  in  the  form  of  a  seric.  .,  llul 
connected  with  the  trigeminal  and  facial  nerves  and  known'^sS 
chary,  spheno-palatine,  otic,  and  submaxillary  ganglia;  and^as  wH 


!  n 


i- 


•  M 


433 


TBS  SYMPATHETIC  SYBim 


I 


be  seen  later,  there  are  probably  some  sympathetic  cells  which  owe 
their  origin  to  the  root  ganglia  of  the  vagus  and  glossopharyngeal 
nerves.  There  is  nothing,  however,  in  the  head  region  corresponding 
to  the  longitudinal  bundles  of  fibers  which  unite  the  various  pionmal 
ganglia  of  the  trunk  to  form  the  ganglionated  cord. 

The  first  distinct  indications  of  the  sympathetic  system  are  to  be 
seen  in  a  human  embryo  of  about  7  mm.    As  the  spmal  nerves 
reach  the  level  of  the  dorsal  edge  of  the  body-cavity,  they  branch, 
one  of  the  branches  continuing  ventrally  in  the  body-wall,  while  the 
other  (Fig  250,  wr)  passes  mesiaUy  toward  the  aorta,  some  of  its 
fibers  reaching  that  structure,  while  others  bend  so  as  to  assume  a 
longitudinal  direction.    These  mesial  branches  represent  the  white 
rami  communicantes,  but  as  yet  no  ganglion  ceUs  can  be  seen  m 
their  course.    The  cells  of  the  posterior  root  ganglia  have  ahready, 
for  the  most  part,  assumed  their  bipolar  form,  but  among  them  there 
may  still  be  found  a  number  of  cells  in  the  neuroblast  condition,  and 
these  (Fig.  250,  3),  wandering  out  from  the  ganglia,  pve  "se  to  a 
column  of  celb  standing  in  relation  to  ;he  white  rami.    At  first  there 
is  no  indication  of  a  segmental  arrangement  of  the  ceUs  of  the  column 
fFie   2"),  but  at  about  the  seventh  week  such  an  arrangement 
makes  its  appearance  in  the  cervical  region,  and  later,  extends 
posteriorly,  until  the  column  assumes  the  form  of  the  ganghonated 

"^  This  origin  of  the  ganglionated  cord  from  cells  migrating  out 
from  the  posterior  root  gangUa  has  been  described  by  various 
authors,  biit  recently  the  origin  of  the  cells  has  been  earned  a  step 
further  back,  to  the  mantle  layer  of  the  central  nervous  system 
(Kuntz)  Indifferent  cells  and  neuroblasts  are  said  to  wander  out 
from  the  waUs  of  the  medullary  canal  by  way  of  both  the  posterior 
and  anterior  nerve  roots  and  it  is  claimed  that  these  are  the  cells  that 
give  rise  to  the  ganglionated  cord  in  the  manner  just  desCTibed. 
Before,  however,  the  segmentation  of  the  ganglionated  cord  be- 
comes marked,  thickenings  appear  at  certain  regions  of  the  ceU 
column,  and  from  these,  bundles  of  fibers  may  be  seen  extending 
ventraUy  toward  the  viscera.    The  thickenings  represent  certainol 


TSm   SniFAIBETIC  SYSTEH  ^« 

the  prevertebral  ganglia,  and  later  cells  wander  out  from  them  and 
take  a  position  in  front  of  the  aorta.  In  an  embryo  of  10.2  mm.  two 
ganghomc  masses  (Fig.  251,  pc)  occur  in  the  vicinity  of  the  origin 


4.  ^  J^^^TTiS^iTiTK'""^  '"'^-  »^.8"fi<»i'^  ™«»  "printing 

of  the  vitelline  artery  (am),  one  lying  above  and  the  other  below 
that  vessel;   these  masses  represent  the  ganglia  of  the  cceliac 


434 


UTEIATUBE 


plexus  and  have  separated  somewhat  from  the  ganglionated  cord, 
the  fiber  bundles  which  unite  the  upper  mass  with  the  cord  represent- 
ing the  greater  and  lesser  splanchnic  nerves  (j^)  ,  while  that  connected 
with  the  lower  mass  represents  the  connection  of  the  cord  with  the 
superior  mesenteric  ganglion.  Lower  down,  in  the  neighborhood 
of  the  umbilical  arteries,  is  another  enlargement  of  the  cord  (bg), 
which  probably  represents  the  inferior  mesenteric  and  hypogastric 
ganglia  which  have  not  yet  separated  from  the  cell  column. 

With  the  peripheral  ganglia  the  conditions  are  slightly  different, 
in  that  they  are  formed  very  largely,  if  not  exclusively,  from  cells 
that  migrate  from  the  walls  of  the  hind-b.ain  by  way  of  the  vagus 
nerves  (Fig.  251).  In  this  way  the  ganglia  of  the  myenteric,  pul- 
monary and  cardiac  plexuses  are  formed,  though  in  the  case  of  the 
last  named  it  is  probable  that  contributions  are  also  received  from 
the  ganglionated  cord. 

The  elongated  courses  of  the  cardiac  sympathetic  and  splanchnic 
nerves  in  the  adult  receive  an  explaWtion  from  the  recession  of  the  heart 
aad  diaphragm  (see  pp.  239  and  332),  the  latter  process  forcing  down- 
ward the  coeliac  plexus,  which  originally  occupied  a  position  opposite 
the  region  of  the  ganglionated  cord  from  whidi  the  splanchnic  nerves 
arise. 

As  regards  the  cephalic  sympathetic  ganglia,  the  observations 
of  Remak  on  the  chick  and  Kfilliker  on  the  rabbit  show  that  the 
ciliary,  sphenopalatine,  and  otic  ganglia  arise  by  the  separation  of 
cells  from  the  semilunar  (Gasserian)  ganglion,  and  from  their  adult 
relations  it  may  be  supposed  that  the  cells  of  the  submaxillary  and 
sublingual  ganglia  have  similarly  arisen  from  the  geniculate  ganglion 
of  the  facial  nerve.  Evidence  has  also  been  obtained  from  human 
embryos  that  sympathetic  cells  are  derived  from  the  ganglia  of  the 
vagus  and  glossopharyngeal  nerves,  but,  instead  of  forming  distinct 
ganglia  in  the  adult,  these,  in  all  probability,  associate  themselves 
with  the  first  cervical  ganglia  of  the  ganglionated  cord. 


LITERATURE. 

C.  R.  Bakdein:  "The  Growth  and  Histogenesis  of  the  Cerebrospinal  Nerves 
'  Amtr.  Jown.  Anal.,  a,  1903. 


LITEKATUKE 


4^5 


S.  R^JAi:  "No««««  Ol>«n«ion.  «r  I'evolution  d«  nn.reblwt.  .,«  „„tau« 

«™qu»^«.r  rhypou*.  „«.^,u,u.  d.  H„«.-H..d," !;::"!":!?;;: 
'•  ''^j:::<^xz^  """°  •""'"" "" "'"°  ""■"-'-'•"  -"'•  "'■»•" 

^'  ^^^.T^  7''  '^'  ^''""'='"''"«  1"  «"»«.  Hi™commi«u™,  und  die 

I  t!^,^oTTT'\ '"^'-  ""^-  ""*•/■  ^'^-  "^ o^*"..  xv'Tr 

"•  "".rz::  ^rrv'r^r"'  °"  *-  °""°''"-  ■>'  >■«'"»'- ''— .■ 

Hf^    Z"  G«cluch«  dcs  Gehims  »wie  d.r  centralen  und  peripherachlnNerven 
W.  tts:  "Die  Formcntwickdung  de»  m«,Khlich«.  Vorderhim.  vom  Ende  d«  en.«, 

*■  ^p^'!^"!"""'"^  ""  ""°""'^'«"  """^  -^^^  ler  er«„  Mona.. ." 

*■  'S'id/'?i:**.^'r  7  f  «-— ™y»'™  bei  WirbelUueren,"  Aikan^. 
W   H^  '^'"t'-Sf'hmchm  Ge^eUsch.,  MM.-Pkysik.  Clo^se.  xvm,  ,893. 

M^i-hin  ,?••«' :'"En.wickclu„g  d«  BaucluympaUucu,  bdm  Hflhnchen  und 

t^^^r         ^"■'"'' »»''  ''"^'  Spi"l  N«rv«  of  M«,idU:  A  Contribution  upon 

aJ^™„  «■,    ^%^."°',"  ''""''  -'I  Cutan„us  Sen«H,rg.n,  of  U^e  North 
Amenom  SUurotd  F«h«,"  y«™.  „/c«,;>.  Neurol..  «,  ,90. 


Mil 


'I 


4i6  UmATDU 

A.  Koim:  "The  i«l«  of  the  Vifl  In  the  Devclapmeiit  tt  the  Sjrmpethetk  Nermw 

Syitcm,"  Ami.  Anmif,  xxxv,  1909. 
A.  Hmrm  "The  Development  of  the  Sympethetk  Nenroiu  Syetem  In  Mwnnub, 

Jmtm.  Campar.  Nmmoi.,  xx,  1910. 
M.  VON  L»NHom««:  "Die  Entwkfcelung  der  GengllnienUgen  bei  dem  meBK*Bchen 

Embiyo,"  ArMvJV  Ant  mdPhysM.,  Ant.  AUk.,  1891. 

F.  Maichano:  "Ueber  die  Entwlckehing  da  Bilkene  Im  menichlkhen  Gdiini," 

Arckhfir  mikrmi.  Ant.,  xxxvB,  1891. 
V.  VOH  MiBAUOVici:  "EntwlckdungigeKhfchte  dee  Gehinu,"  Leipilg,  1877- 
A.  D.  Ohodi:  "Ueber  die  Entwickelung  det  qrmpnthiichen  Nenreneyeteme,"  Ankh 

f»  nutrosU.  Ant.,  xxvn,  1886. 

G.  Kmroa:  "  Des  McnKhenhini,"  Stockhohn,  i89'- 

A.  ScBAPUi:  "Die  frilhesten  DiSerenziiungevorgtnfe  Im  Centml-nerven-ejretem,' 

Ardmfat  Entmckhmfsmeckami,  v,  1897. 
G.  L.  9rlI«TE»:  "  The  Development  of  the  Cnnial  and  Sptonl  Nervee  hi  the  OcdplU 

Region  o(  the  Human  Embryo,"  Anur.  Jam.  Ant.,  TV,  1904- 
O.  S.  SiaoNo:  "The  Cranial  Nervee  of  Amphibia,"  Jivrnal  of  tlarfkt^  X,  189S. 
R.  WiAsauc:  "Die  Herkunft  dee  Myelin.,"  Arckiv  ftr  Entwickbmfsnmkma,  \\ 

1898. 
E.  Zocimkanpl:  "Zur  Entwicklung  dee  Balkena,"  ArMlm  am  nturtl.  Inst.  Win. 

xvn, 1909. 


CHAPTER  XVI. 

THE  DEVELOPMEHT  OF  THE  OROARS  OF 
SPECIAL  SEirSB. 

like  the  cells  of  the  central  nervous  system,  the  sensory  cells 
are  all  of  ectodermal  origin,  and  in  lower  animals,  such  as  the  earth- 
worm for  instance,  they  retain  their  original  position  in  the  ecto- 
dermal epithelium  throughout  life.  In  the  vertebrates,  however, 
the  majority  of  the  sensory  cells  relinquish  their  sup^ficial  position 
and  «nk  more  or  less  deeply  into  the  subjacent  tissues,  being  repre- 
sented by  the  posterior  root  ganglion  cells  and  by  the  sensory  cells 
of  the  special  sense-organs,  and  it  is  only  in  the  olfactory  organ  that 
0>e  onginal  condition  is  retained  Those  cells  which  have  with- 
drawn from  the  surface  receive  sUmuli  only  through  overlying  cells, 
and  m  certain  cases  these  transmitting  cells  are  not  specially  differ- 
entiated, the  terminal  branches  of  the  sensory  dendrites  e  ding 
among  ordinary  epithelial  cells  or  in  such  structures  as  the  Pacinian 
bodies  or  the  end-bulbs  of  Krause  situated  beneath  undifferentiated 
epithdium.  In  other  cases,  however,  certaii.  specially  modified 
superficial  cells  serve  to  transmit  the  stimuli  to  the  peripheral  sensory 
neurones,  forming  such  structures  as  the  hair-cells  of  the  auditory 
epithehum  or  the  gustatory  cells  of  the  taste-buds. 

Thus  three  degrees  of  differentiation  of  the  special  sensory  cells 
may  be  recognized  and  a  classification  of  the  sense-organs  may  be 
made  upon  this  basis.  One  organ,  however,  the  eye,  cannot  be 
brought  mto  such  a  classification,  since  its  sensory  cells  present 
certam  developmental  peculiarities  which  distinguish  them  from 
those  of  all  other  sense-organs.  Embryologically  the  retina  is  a 
portion  of  the  central  nervous  system  a=d  not  a  peripheral  organ 
and  hence  it  will  be  convenient  to  arrange  the  other  sense-organs 
4*7 


4»8 


THE   OLrACTOCV  OKOANt 


^& 


according  to  the  cla»»ificalion  indicated  and  to  discus*  the  history 
of  the  eye  at  the  close  of  the  chapter. 

The  DeTelopmrat  of  th«  Olfactory  Organ— The  general 
development  of  the  nasal  fossa,  the  Lpithelium  of  which  contains  the 
olfactory  sense  cells,  has  already  been  described  (pp.  99  and  283), 
as  has  also  the  development  of  the  olfactory  lobes  of  the  brain 
(p.  406),  and  there  remains  for  consideration  here  merely  the  forma- 
tion of  the  olfactory  nerve  and  the  development  of  the  rudimentary 
organ  of  Jacobson. 

The  Olfactory  Nerve.— Weiy  diverse  results  have  been  obtained  by 
various  observers  of  the  development  of  the  olfactory  nerve,  it  having 
been  held  at  different  times  that  it  was  formed  by  the  outgrowth  of 
fibers  from  the  olfactory  lobes  (Marshall),  from  fibers  which  arise 
partly  from  the  olfactory  lobes  and  partly  from  the  olfactory  epithe- 
lium (Beard),  from  the  cells  of  an  olfactory  ganglion  originally  derived 
from  the  olfactory  epithelium  but  later  separating  from  it  (His), 
and,  finally,  that  it  was  composed  of  the  prolongations  of  certain 
cells  situated  and,  for  the  most  part  at  least,  remaining  permanently 
in  the  olfactory  epithelium  (Disse).  The  most  recent  observations  on 
the  structure  of  the  olfactory  epithelium  and  nerve  indicate  a  greater 
amount  of  probability  in  the  last  result  than  in  the  others,  and  the 
description  which  follows  will  be  based  upon  the  observations  of  His, 
modified  in  conformity  with  the  results  obtained  by  Disse  from  chick 
embryos. 

In  human  embryos  of  the  fourth  week  the  cells  lining  the  upper 
part  of  the  olfactory  pits  show  a  distinction  into  ordinary  epithelial 
and  sensory  cells,  the  latter,  when  fully  formed,  being  elongated 
cells  prolonged  peripherally  into  a  short  but  narrow  process  which 
reachfi  the  surface  of  the  epithelium  and  proximally  gives  rise  to 
an  axis-cylinder  process  which  extends  up  toward  and  penetrates  the 
tip  of  the  olfactory  lobe  to  come  into  contact  with  the  dendrites  of 
the  first  central  neurones  of  the  olfactory  tract  (Fig.  252).  These 
cells  constitute  a  neuro-epithelium  and  in  later  stages  of  develop- 
ment retain  their  epithelial  position  for  the  most  part,  a  few  of  them, 
however,  withdrawing  into  the  subjacent  mesenchyme  and  becoming 


THK   OtrACTOlY  OKOUta  4,g 

Wpolar,  their  peripheral  p«,lo„gatio„s  ending  freely  among  the  cell, 
of  the  olfactonr  epithelium.    The«  bipolar  cells  resemWe  cl<^ly 

Xlid  cI.""  '""^"^""'^ '""'"°"  '"'*'^"  """« ■""»  '"--- 
T-fe  Or,o„  <,/  Jacohsm.-ln  embryo,  of  three  or  four  month,  a 


F.O.  a5».-D.Ao««  a.usT«rLvo  thxr.:^x.o»s  o,  the  F,be«  o,  the  Ol„cto« 

Ttiinr'll'"'''  'rt^'°^"°"  °f  'he  epithelium  covering  the  lower 
antenor  portion  of  the  median  septum  of  the  nose  can  readily  be 

ndt.  l^''^'""^'^  ^°"^'="^'l  i"to  a  slender  pouch,  3  to  5  mm.  long, 
endang  blmdly  at  Us  posterior  extremity  and  opening  at  its  other  end 


43° 


TBE   0KGAN8  OF  TASTE 


into  the  nasal  cavity.  Ite  lining  epithelium  resembles  that  of  the 
respiratory  portion  of  the  nasal  cavity,  and  there  is  developed  in  the 
connective  tissue  beneath  its  floor  a  slender  plate  of  cartilage,  dis- 
tinct from  that  forming  the  septum  of  the  nose. 

This  organ,  which  may  apparently  undergo  degeneration  in  the 
adult,  and  in  some  cases  completely  disappears,  appears  to  be  the 
representative  of  what  is  known  as  Jacobson's  organ,  a  structure 
which  reaches  a  much  more  extensive  degree  of  development  in  many 
of  the  lower  mammals,  and  in  these  contains  in  its  epithelium  sensor) 
cells  whose  axis-cylinder  processes  pass  vrith  those  of  the  olfactorj 
sense  cells  to  the  olfactory  bulbs.  In  man,  however,  it  seems  to  be  a 
rudimentary  organ,  and  no  satisfactory  explanation  of  its  function 
has  as  yet  been  advanced. 

The  olfactory  neuro-epithelium,  considered  from  a  comparative 
standpoint,  seems  to  have  been  derived  from  the  system  of  lateral 
line  organs  so  highly  developed  in  the  lower  vertebrates  (Kupffer). 
In  higher  forms  the  system,  wUch  is  cutaneous  in  character,  has 
disappeared  except  in  two  regions  where  it  has  become  highly 
specialized.  In  one  of  these  regions  it  has  given  rise  to  the  olfactory 
sense  cells  and  in  the  other  to  the  similar  cells  of  the  auditory 
apparatus. 

The  Organs  of  Touch  and  Taste.— Little  is  yet  known  con- 
cerning the  development  of  the  various  forms  of  tactile  organs,  which 
belong  to  the  second  class  of  sensory  organs  described  above. 

The  Organs  of  Toite.— The  remaining  organs  of  special  sense 
belong  to  the  third  class,  and  of  these  the  organs  of  taste  present  in 
many  respects  the  simplest  condition.  They  are  developed  prin- 
cipally in  connection  with  the  vallate  and  foliate  papilUc  of  the 
tongue,  and  of  the  former  one  of  the  earliest  observed  stages  has 
been  found  in  embryos  of  9  cm.  in  the  form  of  two  ridges  of  epi 
dermis,  lying  toward  the  back  part  of  the  tongue  and  inclined  to  one 
another  in  such  a  manner  as  to  form  a  V  with  the  apex  directed 
backward.  From  these  ridges  solid  downgrowths  of  epidermis 
into  the  subjacent  tissue  occur,  each  downgrowth  having  the  form 
of  a  hollow  truncated  cone  with  its  basal  edge  continuous  with  the 


XBE   INIXRKAI,  EAS 


431 


sup«rficw^  epideraus  (Fig.  253,  A).  In  later  stages  lateral  out- 
growths develop  from  the  deeper  edges  of  the  cone,  and  about  the 
same  hme  c  efts  app«ir  in  the  substance  of  the  original  downgrowths 
(Fig.  2S3.  B)  and,  uniting  together,  finally  open  to  the  surface,  form- 

"""ir^'u-T"^*"*  *  P^P"'*  (^'8-  'S3,  C).  The  lateral  out- 
growths, which  are  at  first  solid,  also  undergo  an  axial  degeneration 
and  become  converted  into  the  glands  ofEbner  (6),  which  open  into 
the  teench  ne«  its  floor.  The  various  papilte  which  occur  in  the 
adu^t  do  not  develop  simultaneously,  but  their  number  increases 
with  the  age  of  the  fetus,  and  there  is,  moreover,  considerable 
variation  in  the  Ume  of  their  development. 

The  taste-buds  are  formed  by  a  differentiation  of  the  epithelium 
which  covers  the  papilte,  and  this  differentiation  appears  to  stand 


'"'■  '27^?'°"'"  *r'?'™"'  ^  D™i.opkent  oka  vallate  Papilla. 
o,  Valley  .urrounding  the  papiUa;  4,  ron  Ebner's  gUnd.-(CroJ„,.) 

in  intimate  relation  with  the  penetration  of  fibers  of  the  glosso- 
pharyngeal nerve  into  the  papUte.  The  buds  form  at  various  places 
upon  the  papilte,  and  at  one  period  are  especially  abundant  upon 
th^  fi^  surfaces,  but  in  the  later  weeks  of  intrauterine  life  these 

'"fl^^l  u  "^^"^  degeneration  and  only  those  upon  the  sides 
of  the  trench  persist,  as  a  rule. 

The  foliate  papilte  do  not  seem  to  be  developed  until  some  time 
after  the  arcumvallate,  being  entirely  wanting  in  embryos  of  four 
and  a  half  and  five  months,  although  plainly  recognizable  at  the 
seventh  month. 

Th.  DeTelopment  of  the  Ear.-It  is  customary  to  describe  the 
mammalian  ear  as  consisting  of  three  parts,  known  as  the  inner, 
middle,  and  outer  ears,  and  this  division  is,  to  a  certain  extent  at 


11 

r 


433 


TKE   INTERNAL  EAS 


least,  confirmed  by  the  embryonic  development.  The  inner  ear, 
which  is  the  sensory  portion  proper,  is  an  ectodermal  structure,  which 
secondarily  becomes  deeply  seated  in  the  mesodermal  tissue  of  the 
head,  while  the  middle  and  outer  ears,  which  provide  the  apparatus 
necessary  for  the  conduction  of  the  sound-waves  to  the  inner  ear, 
are  modified  portions  of  the  anterior  branchial  arches.  It  will  be 
convenient,  accordingly,  in  the  description  of  the  ear,  to  accept 
the  usually  recognized  divisions  and  to  consider  first  of  all  the 
development  of  the  inner  ear,  or,  as  it  is  better  termed,  the  otoeyst. 
The  Development  of  the  Olocyst.~ln  an  embryo  of  2.4  mm.  a 
pair  of  pits  occur  upon  the  surface  of  the  body  about  opposite  the 
middle  portion  of  the  hind-brain  (Fig.  254,  A).  The  ectoderm 
lining  the  pits  is  somewhat  thicker  than  is  the  neighboring  ectoderm 


A  ^  B 

Fio.  154.— TiANSviiSE  Section  Passdio  thkodoh  thi  Otocvst  (of)  or  Embryos  of 

(.4)  J.4  MM.  AND  (B)  4  MM.— (H«.) 

of  the  surface  of  the  body,  and,  from  analogy  with  what  occurs  in 
other  vertebrates,  it  seems  probable  that  the  pits  are  formed  by  the 
invagination  of  localized  thickenings  of  the  ectoderm.  The  mouth 
of  each  pit  gradually  becomes  smaller,  until  finally  the  invagination 
is  converted  into  a  closed  sac  (Fig.  254,  B),  which  separates  from  the 
surface  ectoderm  and  becomes  enclosed  within  the  subjacent  meso- 
derm. This  sac  is  the  otoeyst,  and  in  the  stage  just  described, 
found  in  embryos  of  4  mm.,  it  has  an  oval  or  more  or  less  spherical 
form.  Soon,  however,  in  embryos  of  6.9  mm.,  a  prolongation 
arises  from  its  dorsal  portion  and  the  sac  assumes  the  form  shown  in 
Fig.  255,  A;  this  prolongation,  which  is  held  by  some  authors  to  be 
the  remains  of  the  stalk  which  originally  connected  the  otoeyst  sac 


IHE    DITEHNAI   EAR  .,, 

433 
vnth  the  surface  ectoderm,  represents  the  ductus  endolymphaticus 
and,mcreasmg  .„  length,  it  soon  becomes  a  strong  cKS 
r„T'  7^'"'  ^-iderably  beyond  the  remaini^ng  port^oTol 
^otocyst  (Fig.  ,ss,  B).  In  embryos  of  about  lo..  mm  th^sac 
b^ns  to  show  certain  other  irregularities  of  shape  (Fig.  ."5.  B  S 

a^tus  ^h        ;P^'^"'  """'  °'  °"»"  "f  the  ductuf  endo  ymph- 
aucus  three  folds  make  their  appearance,  representing  the  W 


F.O.  ,55.-R.««,T.„C„ON  O,  ™  OTOC««  or  E-BHVO  o,  U)  6.,  «. .™  (B, 

c^cufar  d«.to,  and  as  they  increase  in  size  the  opposite  walls  of  the 

Ztd°r°  °'  TV""  ^°""=  "^''''"'  ^"-''^  finally  b::om 
otre,^„f  T";*         '''  ''^'^  °'  "'^  ^°'^  ''^  "  '^centic  canal,  at 
one  end  of  which  an  enlargement  appears  to  form  the  amMaia.    The 

inThe  I^  T.H   !  '  ""^- '"'°  "'"•'"^  "'"'^  P'""  -"-"at  earlier 
m  the  cases  of  the  two  vertical  than  in  that  of  the  horizontal  duct,  as 


1  it 


434 


Tint  DITBSMAL  EAR 


may  be  wen  from  Fig.  356,  which  represents  the  condition  occurring 
in  an  embryo  of  13.5  mm. 

A  short  distance  below  the  level  at  wWch  the  canals  commumcate 
with  the  remaining  portion  of  the  otocyst  a  constriction  appears, 
indicating  a  separation  of  the  otocyst 
into  a  more  dorsal  portion  and  a  more 
ventral  one.    Later,  the  latter  begjns 
to  be  prolonged  into  a  flattened  canal 
which,  as  it  elongates,  becomes  coiled 
upon  itself  and  also  becomes  separated 
by  a  constriction  from  the  remaining 
portion  of   the   otocyst   (Fig.    257)- 
This  canal  is  the  ducttis  cocUearis 
(scala  media  of  the  cochlea),  and  the 
remaining  portion  of  the  otxyst  sub- 
sequently becorr\tiS  divided  by  a  con- 
striction into  the«frtc«/«J,  with  which 
the  semicircular  ducts  are  connected, 
and  the  sacculus.    The  constriction 
which  separates  the  cochlear  duct  from 
the  sacculus  becomes  the  ductus  rt- 
Mttefu,  while  that  between  the  utri- 
culus  and  sacculus  is  converted  into 
a  narrow  canal  with  which  the  ductus 
endolymphaticus  connects,  and  hence 
it  is  that,  in  the  adult,  the  connection 
between  these  two  portions  of  the 
otocyst  seems  to  be  formed  by  the 
ductus  dividing  prorimally  into  two 
limbs,  one  of  which  is  connected  with 
the  utricle  and  the  otiier  with  the  saccule. 

When  first  observed  in  the  human  embryo  the  auditory  ganglion 
is  closely  associated  with  the  geniculate  ganglion  of  the  seventh 
nerve  (Fig.  255,  R),  the  two,  usually  spoken  of  as  the  acusUco-faciahs 
gan^on,  fomii.«  a  mass  of  cells  lying  in  close  cont«:t  with  the 


Fio.  a56.— RicoNsmocnoN  of 
THE  Otocyst  or  ak  Ekbmo  or 
13.5  im. 

CO,  Cochlem;  it,  endolymphatoc 
duct;  «,  semicircular  duct. — (H«. 
/r.) 


THE  INTKBNAL  XAX 


435 


antmor  wall  of  the  otocyst.  The  origin  of  the  ganglionic  mass  has 
not  yet  been  traced  in  the  mammalia,  but  it  has  been  observed  that 
m  cow  embryos  the  geniculate  ganglion  is  connected  with  the  ecto- 
derm at  the  dorsal  end  of  the  first  branchial  cleft  (Froriep),  and  it 
may  perhaps  be  regarded  as  one  of  the  epibranrhial  placodes  (see  p. 
417).  and  in  the  lower  vertebrates  a  union  of  the  ganglion  with  a 
suprabranchial  placode  has  been  observed  (Kup£fer),  this  union 


Fio.  JS7-RK»HST»nciioN  ot  iHi  Otocvst  of  an  Ehbko  ow  ,0  MM,  noHT  vnw 

«■  S-SlfT"  "?*?  "*  "iP*?°5  "^  paiterior  Mmidrcular  ducts;  eg,  cochlear  KamrUon- 
«>,  cochlea;  A.  endolymphatic  duct;  j,  sacculuj;  sdl,  sdp,  and  s(k  Uter^^terSS  S 
supenor  Kmidrcular  ducts;  u.  utriculus;  vf,  v^tibiitJg^inl-^trt^  "^ 

indicating  the  origin  of  the  auditory  ganglion  from  one  or  more  of 
the  ganglia  of  the  lateral  line  system. 

At  an  early  stage  in  the  human  embryo  the  auditory  ganglion 
shows  indications  of  a  division  into  two  portions,  a  more  dorsal  one, 
which  represents  the  future  ganglion  vestibuhre,  and  a  ventral  one 
the  ganglion  cochleare.  The  ganglion  cells  become  bipolar,  in  which 
condidon  they  remain  throughout  life,  never  reaching  the  T-shaped 
condition  found  in  most  of  the  other  peripheral  cTcbro-spinal  gang- 
lia.   One  of  the  prolongations  of  each  cell  is  directed  centrally  to 


11 

Ijl 


«■;■■ 


:  a 


436 


12E  QITEBNAL  EAR 


form  a  fiber  of  the  auditory  nerve,  while  the  other  penetrates  the  wall 
of  the  otocyst  to  enter  into  relations  with  certain  specially  modified 
cells  which  differentiate  from  its  lining  epithelium. 

In  the  earliest  stages  the  ectodermal  lining  of  the  otocy.:  b 
formed  of  similar  columnar  cells,  but  later  over  the  greater  part  of 
the  surface  the  cells  flatten  down,  only  a  few,  aggregated  together  to 


Fio.  158.— The  Rioht  Inteknal  Ear  of  an  Embeyo  of  Six  Months. 
en,  a,  and  cp,  Superior,  lateral,  and  posterior  semicircular  ducts;  a,  crista  acusUca; 
it,  endolymphatic  duct;  U,  spiral  ligament;  mi,  basilar  membrane;  mj  and  nw,  macula 
acustica  sacculi  and  utriculi;  rb,  basilar  branches  of  the  cochlear  nerve.— tK«<»»«.) 

form  patches,  retaining  the  high  columnar  form  and  developing  hair 
like  processes  upon  their  free  surfaces.    These  are  the  sensory  cells 
of  the  ear.     In  the  human  car  there  are  in  all  six  patches  of  thesu 
sensory  cells,  an  elongated  patch  {aisla  ampullaris)  in  the  ampulla  of 
each  semicircular  canal  (Fig.  258,  cr),  a  round  patch  (macula  acus 


THE   INTERNAL  EAB 


437 


/•CO,  m«)  in  the  utriculus  and  another  (ms)  in  the  sacculus,  and, 
finally,  an  elongated  patch  which  extends  the  entire  length  of  the 
scala  media  of  the  cochlea  and  forms  the  sensory  cells  of  the  spiral 
mrgan  of  Corn.  The  cells  of  this  last  patch  are  connected  with  the 
fibers  from  the  cochlear  ganglion,  while  those  of  the  vestibular 
ganghon  pass  to  the  cristse  and  macula. 

In  connection  with  the  spiral  organ  certain  adjacent  cells  also 
retain  their  columnar  form  and  undergo  various  modifications, 


a.  M™  r^^f"""  "  ""  COCHUAR  Duct  or  a  Rabbit  EyBEvo  op  55  mm. 
"-  ^^"^^y^-'  "  'o  «.  epithelium  of  cochlear  duct;  .,f.,,  „,embr.„a  tectoria;  V.s.p. 
vein,  I  10  7,  spiral  organ  of  CoTl\.—(Baginsky.) 

giving  rise  to  a  rather  complicated  structure  whose  development  has 
•  been  traced  in  the  rabbit.  Along  the  whole  length  of  the  cochlear 
duct  the  cells  resting  upon  that  half  of  the  basilar  membrane  which  is 
nearest  the  axis  of  the  cochlea,  and  may  be  termed  the  inner  half, 
retain  their  columnar  shape,  forming  two  ridges  projecting  slightly 
mto  the  cavity  of  the  scala  (Fig.  259).  The  cells  of  the  inner  ridge, 
much  the  larger  of  the  two,  give  rise  to  the  memyana  Uctoria, 


438 


1HX  OraCKNAL  XAX 


either  as  a  cuticular  secretion  or  by  the  artificial  adhesion  of  long 
hMT-like  processes  which  project  from  their  free  surfaces  (Ayers). 
The  cells  of  the  outer  ridge  are  arranged  in  six  longitudinal  rows 
(Fig.  259,  1-6);  ♦•»<»«  °*  **  innermost  row  (i)  develop  haurs  upon 
their  free  surfaces  and  form  the  inner  hair  cells,  those  of  the  next  two 

rows  (a  and  3)  gradually  be- 
;'.■«'  "  come  transformed  on' their  ad- 

jacent surfaces  into  chitinous 
substance  and  form  the  rods  of 
Corti,  while  the  three  outer  rows 
(4  to  6)  develop  into  the  outer 
hair  cells.  It  is  in  connection 
with  the  hair  cells  that  the  per- 
ipheral prolongations  of  the  cells 
of  the  cochlear  gan^ion  ter 
Fio.  260.— TuANsyriM  Section  niinate,  and  since  these  hair  cells 
THKOnOH  A  SEmcaccLAii    Ddct    01  a  '  .   .  ,. 


„-  -  -c 


are  arranged  m  rows  extending 
the  entire  length  of  the  cochlear 


Rabbr  Embryo  ot  Twinty-jodb  Day*. 
c,  Periotic  cartilage;  ef,   6biou»  mem- 
bram  beneath  the  epithelium  of  the  canal;  "i.         i       •    j 

f,  perichondrium;  s,  apongr  tiaaue.— (Von    duct,  the  ganglion  also  IS  drawn 
kSor.)  out  into  a  spiral  foUowuig  the 

coils  of  the  cochlea,  and  hence  is  sometimes  termed  the  spiral 
ganglion. 

While  the  various  changes  described  above  have  been  taking 
place  in  the  otocyst,  the  mesoderm  surrounding  it  has  also  been 
undergoing  development.  At  first  this  tissue  is  quite  uniform  in 
character,  but  later  the  cells  immediately  surrounding  the  otocyst 
condense  to  give  rise  to  a  fibrous  layer  (Fig.  260,  ep),  while  more 
peripherally  they  become  more  loosely  arranged  and  form  a  some- 
what gelatinous  layer  (i),  and  still  more  peripherally  a  second  fibrous 
layer  is  differentiated  and  the  remainder  of  the  tissue  assumes  a 
character  which  indicates  an  approaching  conversion  into  cartilage. 
The  further  history  of  these  various  layers  is  as  follows:  The  inner 
fibrous  layer  gives  rise  to  the  connective-tissue  wall  which  supports 
the  ectodermal  lining  of  the  various  portions  of  the  otocyst;  the 
gelatinous  layer  undergoes  a  degeneration  to  form  a  lymph-like 


IHI  IMTEIMAL  XAX 


439 


fluid  known  as  the  penlymph,  the  space  occupied  by  the  fluid  being 
the  pailymphatic  space;  the  outer  fibrous  layer  becomes  pen* 
chondnum  and  later  periosteum;  and  the  procartilage  und«ioe8 
chondnfication  and  later  ossifies  to  form  the  petrous  portion  of  the 
temporal  lione. 

The  gelatinous  layer  completely  surrounds  most  of  the  otocyst 

b«  in2  ri!hf  ~"\*°  "'  '"'  '"  '•■*  perilymphatic  space, 

but  in  the  cochlear  region  the  conditions  are  somewhat  different 
In  this  region  the  gelatinous  layer  is  interrupted  along  two  linesi 


Fio.  361.- 


DUOMMIATIC  TSANSVISSI   SECTION  IHIOCOH  *  CoiL  OF  IBI  CocmiA 
SHOWINO  IH.  RKIATION  OF  IBK  SCALA  I-OCHIIA 


'  "^°  °'  ^"^2.  "•  K"^""?  ???1'1««;  h,  I«mi»  spiralfa;  5if,  cochk«  duct-  57" 

an  outer  broad  one  where  the  connective-tissue  wall  of  the  cochlear 
duct  ,s  directly  continuous  with  the  perichondrium  layer,  and  an 
mner  narrow  one  along  which  a  similar  fusion  takes  place  with  the 
^chondnum  of  a  shelf-like  process  of  the  cartilage,  which  later 
ossifies  to  form  the  /a«,«o  spiralis.  Consequently  throughout  the 
cochlear  region  the  perilymphatic  space  is  divided  into  two  compart- 
ments which  communicate  at  the  apex  of  the  cochlea,  while  below 
one,  known  as  the  scala  vestibuli,  communicates  with  the  space 


■i  H 


440 


THE   mODLE   EA> 


surrounding  the  saccule  and  utricle,  and  the  other,  the  uala  lymfam, 
abuts  upon  a  membrane  which  separates  it  from  the  cavity  of  the 
middle  ear  and  represents  a  portion  of  the  outer  wall  of  the  petrous 
bone  where  chondrification  and  ossification  have  failed  to  occur. 
This  membrane  closes  what  appears  in  the  dried  skull  to  be  an 
opening  in  the  inner  wall  of  the  middle  ear,  known  as  the  fenestra 
cochlea  (rotunda) ;  another  similar  opening,  also  closed  by  membrane 
in  the  fresh  skull,  occurs  in  the  bony  wall  opposite  the  utricular 
portion  of  the  otocyst  and  is  known  as  the  fenestra  veslibuli  (ovalis). 

The  Development  of  the  Middle  Ear. — The  middle  ear  develops 
from  the  upper  part  of  the  pharyngeal  groove  which  represents  the 
endodermal  portion  of  the  first  branchial  cleft.  This  becomes 
prolonged  dorsally  and  at  its  dorsal  end  enlarges  to  form  the  tym- 
panic cavity,  while  the  narrower  portion  intervening  between  this 
and  the  pharyngeal  cavity  represents  the  tuba  auditiva  (Eustachian 
tube). 

To  correctly  understand  the  development  of  the  tympanic 
cavity  it  is  necessary  to  recall  the  structures  which  form  its  bound- 
aries. Anteriorly  to  the  upper  end  of  the  first  branchial  pouch 
there  is  the  upper  end  of  the  first  arch,  and  behind  it  the  correspond- 
ing part  of  the  second  arch,  the  two  fusing  together  dorsal  to  the 
tympanic  cavity  and  forming  its  roof.  Internally  the  cavity  is 
bounded  by  the  outer  wall  of  the  cartilaginous  investment  of  the 
otocyst,  while  externally  it  is  separated  from  the  upper  part  of  the 
ectodermal  groove  of  the  first  branchial  cleft  by  the  thin  membrane 
which  forms  the  floor  of  the  groove. 

It  has  been  seen  in  an  earlier  chapter  that  the  axial  mesoderm 
of  each  branchial  arch  gives  rise  to  skeletal  structures  and  muscles. 
The  axial  cartilage  of  the  ventral  portion  of  the  first  arch  is  what  is 
known  as  Meckel's  cartilage,  but  in  that  portion  of  the  arch  which 
forms  the  roof  and  anterior  wall  of  the  tympanic  cavity,  the  cartilage 
becomes  constricted  to  form  two  masses  which  later  ossify  to  form  the 
malletis  and  incus  (Fig.  262,  m  and  t),  while  the  muscular  tissue  of 
this  dorsal  portion  of  the  arch  gives  rise  to  the  tensor  tympani.  Simi- 
larly, in  the  case  of  the  second  arch  there  is  to  be  found,  dorsal  to 


TBX   lODDUE   EAB 


441 


the  extremity  of  the  cartilage  which  forms  the  styloid  process  of  the 
adult,  a  narrow  plate  of  cartilage  which  forms  an  investment  for 
the  facial  nerve  (Fig.  ,62,  VIl),  and  dorsal  to  this  a  ring  of  cartilage 
(rt)  which  surrounvis  a  small  stapedial  artery  and  represenU  the 
stapes. 

It  has  been  found  that  in  the  rabbit  the  mass  of  cells  from  which 
the  supes  is  formed  is  at  its  first  appearance  quite  independent  of 
the  second  branchial  arch  (Fuchs),  and  it  has  been  held  to  be  a 


Fio.  >6i.- 


-S.«.D«o«MMAT.c  View  of  th«  AcDm,.v  Ossicles  of  an  Emeyo  of 

olX  WEEKS. 


i.  i^^i?;:^rs;:cS{'ssr:^  ^^J"te  - -^^^Sj 


derivative  of  the  mesenchyme  from  which  the  periotic  capsule  is 
formed.  In  later  stages,  however,  it  becomes  connected  with  the 
cartilage  of  the  second  branchial  arch,  as  shown  in  Fig,  262,  and 
It  is  a  question  whether  this  connection,  which  is  transitory  'does 
not  really  indicate  the  phylogenetic  origin  of  the  ossicle  from  the 
second  arch  cartilage,  its  appearance  as  an  independent  structure 
being  a  secondary  ontogenetic  phenomenon.  However  that  may 
be,  the  stapedial  artery  disappears  in  later  stages  and  the  stapedius 


44a 


THS   UDDU  MAM 


muscle,  derived  from  the  miuculature  of  the  wcond  branchial  arch 
and  therefore  supplied  by  the  fadal  nerve,  becomes  attached  to  the 
ossicle. 

The  three  ossicles  at  first  lie  embedded  in  the  mesenchyme 
forming  the  roof  of  the  primitive  tympanic  cavity,  as  does  also  the 
chorda  tympani,  a  branch  of  the 
seventh  nerve,  as  it  passes  into  the 
substance  of  the  first  arch  on  the  way 
to  its  destination.  The  mesenchyme 
in  which  these  various  structures  are 
embedded  is  rather  voluminous  (Fig. 
364),  and  after  the  end  of  the  seventh 
month  becomes  converted  into  a  pecu- 
liar spongy  tissue,  which,  toward  the 
end  of  fetal  life,  gradually  degener- 
ates, the  tympanic  cavity  at  the  same 
tinle  expanding  and  wrapping  itself 
around  the  ossicles  and  the  muscles 
attached  to  them  (Fig.  363).  The 
bones  and  their  muscles,  consequently, 
while  appearing  in  the  adult  to  tra- 
verse the  tympanic  cavity,  are  really 
completely  enclosed  within  a  layer  of 
epithelium  continuous  with  that  lining 
the  wall  of  the  cavity,  while  the 
handle  of  the  malleus  and  the  chorda 
tympani  lie  between  the  epithelium  of 
the  outer  wall  of  the  cavity  and  the 
fibrous  mesoderm  which  forms  the 
tympanic  membrane. 
The  extension  of  the  tympanic  cavity  does  not,  however,  cease 
with  its  replacement  of  the  degenerated  spongy  mesenchyme,  but 
toward  the  end  of  fetal  life  it  begins  to  invade  the  substance  of  the 
temporal  bone  by  a  process  similar  to  that  which  produces  the 
ethmoidal  cells  and  the  other  osseous  sinuses  in  connection  with  the 


FlO.  163. — DlAOXAHS  ILLCS- 
TKATINO  THZ  MODE  07  EXTEN- 
SION o»  THE  Tympanic  CAViry 
AXODND  THE  Auohoey  Ossiclis. 

Af,  Malleus;  m,  spongy  rneun* 
chyme;  p^  surface  tA  the  periotic 
capaule;  T,  tympanic  carity. 
The  h*oken  hue  represents  the 
epithelial  lining  of  the  tympanic 
cavity. 


m  EXTHMAL  KAI 


443 


MMl  Mvitiei  (Me  p.  175).  Thii  process  continues  for  some  years 
after  birth  and  resulu  in  the  formation  in  the  mastoid  portion  of  the 
bone  of  the  so<aUed  mcsioid  uUs,  which  communicate  with  the 
tympanic  cavity  and  have  an  epithelial  lining  continuous  with  that 
of  the  cavity. 

The  lower  portion  of  the  diverticulum  from  the  first  pharyngeal 
groove  which  gives  rise  to  the  tympanic  cavity  becomes  converted 
mto  the  Eustachian  tube.  During  development  the  lumen  r>f  the 
tube  disappears  for  a  time,  probably  owing  to  a  proliferati,:  ,  .1  iis 
hmng  epithelium,  but  it  is  re-established  before  birth. 

vJi\^.  •ccount  of  the  development  of  the  ear-bone,  giv-.i  ',.,  it  , 
held  ^  the  malleus  and  incus  are  derivatives  of  the  ll,  i  '.  anci  i.-ii 
(m|jnd.buM  "Oh  «.d  the  stapes  probably  of  the  .eco.d  'l^^"^ 
15^^?  the  general  consensus  of  recent  workers  on  tkt  illiiicu:-  n,.« 

aU  pMuble  modes  of  origm  have  been  at  one  time  or  other  sr    -.t  .| 
The  maUeus  has  very  generaUy  been  accepted  as  cominif  from  the  fist 

ass^ed  It  to  the  second  arch.  But  with  regard  to  the  sUpes  the  .  pi., 
^  ^^'  ^  ""^  "Jf^'^  ,  "  ^  •^'^  '"•^to  b«  derived  ta,m  the  first 
^.^-™.  •  '*^°'*  »^' fro"  "either  one  nor  the  other,  but  from  die 
rSi^H  ;r^  T' »' •^*  ?«°«y»«'  oi;.  fi°»"y.  it  ha.  beek  held  to  have 
acompound  onga,  iu  uA  bemg  a  product  of  the  Kcond  arch  while  it. 
ba«l  pUte  wa.  a  part  of  the  otocyst  investment 

The  DevOofHtnt  of  the  TympaMc  Membrane  and  of  the  Outer 
£»•.— Just  as  the  tympanic  cavity  is  formed  from  the  endodermal 
groove  of  the  first  branchial  cleft,  so  the  outer  ear  owes  its  origin  to 
the  ectodermal  groove  of  the  same  cleft  and  to  the  neighboring  arches. 
The  dorsal  and  most  ventral  portions  of  the  groove  flatten  out  and 
disappear,  but  the  median  portion  deepens  to  form,  at  about  the 
end  of  the  second  month,  a  funnel-shaped  cavity  which  corresponds 
to  the  outer  portion  of  the  external  auditory  meatus.  From  the 
inner  end  of  this  a  solid  ingrowth  of  ectoderm  takes  place,  and  this, 
enlarging  at  its  inner  end  to  form  a  disk-like  mass,  comes  into  reb- 
tion  with  the  gelatinous  mesoderm  which  surrounds  the  maUeus  and 
chorda  tympani.  At  about  the  seventh  month  a  split  occurs  in  the 
disk-like  mass  (Fig.  364),  separating  it  into  an  outer  and  an  inner 


444  THE   EXTERNAL   EAS 

layer,  the  latter  of  which  becomes  the  outer  epithelium  of  the 
tympanic  membrane.  Later,  the  split  extends  outward  in  the 
substance  of  the  ectodermal  ingrowth  and  eventually  unites  with 
the  funnel-shaped  cavity  to  complete  the  external  meatus. 

The  tympanic  membrane  is  formed  in  considerable  part  from 


Fio.  a64. — Horizontal  Section  Passing  through  the  Dorsal  Wall  of  the 

External  Auditory  Meatus  in  an  Embryo  of  4.5  cm. 
c.  Cochlea;  d«,  endolymphatic  duct;  i,  incus;  Is,  transverse  sinus;  m,  malleus;  me. 
meatus  auditorius  eitcnus ;  fiuf,  cavity  of  the  meatus ;  s,  sacculus ;  sc,  lateral  semicircului 
canal;  «/,  posterior  semicircular  canal;  st,  stapes;  (,  tympanic  cavity;  «,  utriculus:  7. 
facial  Tta\t.—(,Siebe*mann.) 

the  substance  of  the  first  branchial  arch,  the  area  in  which  it  occurs 
not  being  primarily  part  of  the  wall  of  the  tympanic  cavity,  but  being 
brought  into  it  secondarily  by  the  expansion  of  the  cavity.  The 
membrane  itself  is  mesodermal  in  origin  and  is  lined  on  its  outor 


IHE   EXTERNAL   EA« 


445 


-TtJ 

:Sl^^  '"  •=""'"'"'"  ^"'^  °"  "'^  ■•"-  "^  -  endodennal 

^1^"  ""?*  ^""~^  "'''''  '■'*  °"«'"  '°  '•«=  portions  of  the  fi«t  and 
^nd  arches  which  bound  the  ent^nce  oUhe  externa  Meatus 
Upon  the  posterior  edge  of  the  first  arth  there  appeaTablt  the 

tubeitles  (F,g.  ,58,  ^,  ,.3)  ^nd  on  the  anterior  edge  of  the  second 


arch  a  corresponding  number  of  tubercles  (4-6)  is  iormed  while  in 
add.t,on    a  longitudinal  furrow,  running  dowi  the  mmUoi'Z 

Z;  """"t  .'  "'^«'  ^'^  '^'"S  P°^'^"°^  ">  'he  tubercles.  Frl 
hese  S.X  ubercles  and  the  ridge  are  developed  the  various  parts  o" 
•he  auncle,  as  n>ay  be  seen  from  Fig.  ^efwhich  represe^  .he 


446  THE  XYX 

transformation  as  described  by  His.  According  to  this,  the  most 
ventral  tubercle  of  the  first  arch  (i)  gives  rise  to  the  tragus,  and  the 
middle  one  ($)  of  the  second  arch  furnishes  the  anHtragus.  The 
middle  and  dorsal  tubercles  of  the  first  arch  (3  and  3)  unite  with  the 
ridge  (e)  to  produce  the  helix,  while  from  the  dorsal  tubercle  of  the 
second  arch  (4)  is  produced  the  anlkeUx  and  from  the  ventral  one  (6) 
the  hbiUe.  More  recent  observations,  however,  seem  to  indicate 
that  the  lobule  is  an  accessory  structure  unreUted  to  the  tubercles 
and  that  the  sixth  tubercle  gives  rise  to  the  antitragus,  while  the 
fifth  is  either  included  in  the  anthelix  or  else  disappears.  It  is 
noteworthy  that  up  to  about  the  third  month  of  development  the 
upper  and  posterior  portion  of  the  helix  is  bent  forward  so  as  to 
conceal  the  anthelix  (Fig.  265,  D) ;  it  is  at  just  about  a  corresponding 
stage  that  the  pointed  form  of  the  ear  seen  in  the  lower  mammals 
makes  its  appearance,  and  it  is  evident  that,  were  it  not  for  the  for- 
ward bending,  the  human  ear  would  also  be  assuming  at  this  stage 
a  more  or  less  pointed  form.  Indeed,  there  is  usually  to  be  found 
upon  the  incurved  edge  of  the  helix,  some  distance  below  the  upper 
border  of  the  auricle,  a  more  or  less  distinct  tubercle,  known  as 
Darwin's  tuberde,  which  seems  to  represent  the  point  of  the  typical 
mammalian  ear,  and  is,  accordingly,  the  morphological  apex  of  the 
pinna. 

I'fThere  seems  to  be  little  room  for  doubt  that  the  otocyst  belongs 
primarily  to  the  system  of  lateral  line  sense-organs,  but  a  discunion  of  this 
intereating  question  would  necessitate  a  consideration  of  details  concern- 
ing the  devetopment  «f  the  lower  vertd>rates  which  would  be  foreign  to 
the  general  plan  of  this  book.  It  may  be  recalled,  however,  that  the 
analysis  of  the  components  of  the  cranial  nerves  described  on  page  415 
refers  the  auditory  nerve  to  the  lateral  line  system. 

The  DeTelopment  of  Ae  Eye. — ^The  first  indications  of  the 
development  of  the  eye  are  to  be  found  in  a  pair  of  hollow  out- 
growths from  the  side  of  the  first  primary  brain  vesicle,  at  a  level 
which  cocresponds  to  the  junction  of  the  dorsal  and  ventral  zonii, 
£^h  eragination  is  directed  at  first  upward  and  backward,  and, 
enlarging  at  its  extremity,  it  soon  shows  a  differentiation  into  a 


IRE  EYE 


447 


A.  tl.e  re.ult  of  tl«>  dep««si«  of  the  lens  ectoderm,  ^e  ««er  wall 


il  11 


448  THE   EYE 

of  the  optic  bulb  becomes  puslwd  inward  toward  the  inner  wall,  and 
this  invagination  continuing  until  the  two  walls  come  into  contact, 
the  bulb  is  transformed  into  a  double-walled  cup,  the  eplic  cup,  in 
the  mouth  of  which  lies  the  lens  (Fig.  268).  The  cup  is  not  perfect, 
however,  since  the  invagination  affects  not  only  the  optic  bulb,  but 
also  extends  medially  on  the  posterior  surface  of  the  stalk,  forming 
upon  this  a  longitudinal  groove  and  producing  a  defect  of  the  ventral 
w»ll  of  the  cup,  known  as  the  ckorioidal  fissure  (Fig.  267).  The 
groove  and  fissure  become  occupied  by  mesodermal  tissue,  and  in 
this,  at  about  the  fifth  week,  a  blood-vessel  develops  which  traverses 


FlO.  267. — RZCONSTEUCTION  OF  THE  BkAIN  OF  AN  EUBKYO  OF  FoUR  WEEKS,  SHOWING 

THE  C^owoiD  Fissure.—  'JTm.) 

the  cavity  of  the  cup  to  reach  the  lens  and  is  known  as  the  arleria 
hyaloidea. 

In  the  meantime  further  changes  have  been  taking  place  in  thi 
lens.  The  ectodermal  depression  which  represents  it  gradually 
deepens  to  form  a  cup,  the  lips  of  which  approximate  and  finally 
meet,  so  that  the  cup  is  converted  into  a  vesicle  which  finally  sepa 
rates  completely  from  the  ectoderm  (Fig.  268),  much  in  the  same 
way  as  the  otocyst  does.  As  the  lens  vesicle  is  constricted  off,  the 
surrounding  mesodermal  tissue  grows  in  to  form  a  layer  betwetn 
it  and  the  overlying  ectoderm,  and  a  split  appearing  in  the  layt  r 


XBE   EYZ 


449 


....     .  .  ""y 

aivides  it  into  an  outer  thiVbpr  n««: i.-  i 


,V^r 


"Mtrm,     lens,  P.  p,g»«,  ^^  «,  r«i„«,  u,^  ^  „^  ^_^^ 

the  cup  condenses  lo  form  a  strong  in«e«»ent  f,>r  -   »t,-  u 
t'mally  co.tinuous  »ith  the  coml  !^1  "  ''^ 

shows  a  d*r«.tiati„  .„eo  an  inr^Tu^?  ^^Zf 
and  an  oute,  dense,  o^  .h,eh  beco-e«  ,*.  Xi. tl^^  "*' 
The  vano.  proce«.  resuJti^  ,„  ^h,  fo^^,.^  ^^  ^^^ 


450 


THE    UlNS 


which  have  thus  been  rapMly  sketche<l,  may  now  be  considered  in 
greater  detail. 

The  Development  of  Ike  Lens.  When  the  lens  vesicle  is  complete, 
it  forms  a  more  or  less  spherical  sac  lying  beneath  the  superficial 
ectoderm  and  containing  in  its  cavity  a  few  cells,  either  scattered 
or  in  groups  (Fig.  268).  These  cells,  which  have  wandered  into 
the  cavity  of  the  vesicle  from  its  walls,  take  no  part  in  the  further 
development  of  the  lens,  but  early  undergo  complete  degeneration 
and  the  first  change  which  is  concerned  with  the  actual  formation 
of  the  lens  is  an  increase  in  the  height  of  the  cells  forming  its  inner 
wall  and  a  thinning  out  of  its  outer  wall  (Fig.  269,  A).  These 
changes  continuing,  the  outer  half  of  the  vesicle  becomes  converted 
into  a  single  layer  of  somewhat  flat  cells  which  persist  in  the  adult 
condition  to  form  the  anterior  epithelium  of  the  lens,  while  the  cells  of 
the  posterior  wall  form  a  marked  projection  into  the  cavity  of  the 
vesicle  and  eventually  completely  obliterate  it,  coming  into  contact 
with  the  inner  surface  of  the  anterior  epithelium  (Fig.  269,  B). 

Thvse  posterior  elongated  cells  form,  then,  the  principal  mass 
of  the  lens,  and  constitute  what  are  known  as  the  lens  fibers.  At 
first  those  situated  at  the  center  of  the  posterior  wall  are  the  longest, 
the  more  peripheral  ones  gradually  diminishing  in  length  until  at 
the  equator  cf  the  lens  they  become  continuous  with  and  pass  into  tin- 
anterior  epithelium.  As  the  lens  increases  in  size,  however,  the 
most  centrally  situated  cells  fail  to  elongate  as  rapidly  as  the  more 
peripheral  ones  and  are  pushed  in  toward  the  center  of  the  lens,  the 
more  peripheral  fibers  meeting  below  them  along  a  line  passhig 
across  the  inner  surface  of  the  lens.  The  disparity  of  growth  con 
tinuing,  a  similar  sutural  line  appears  on  the  outer  surface  beneath 
the  anterior  epithelium,  and  the  fibers  become  arranged  in  conci  n- 
tric  layers  around  a  central  core  composed  of  the  shorter  fibers. 
In  the  human  eye  the  line  of  suture  of  the  peripheral  fibers  becomes 
bent  so  as  to  consist  of  two  limbs  which  meet  at  an  angle,  and  fnim 
the  angle  a  new  sutural  line  develops  during  embryonic  life,  so  that 
the  suture  assumes  the  form  of  a  three-rayed  star.     In  later  life  ihe 


THE   UN8 


4SI 


Fig.  26,.— SrenoKi  tboooh  tue  Lns  M)  of  Htiwai.,  it. 


4Sa  THE    LIN8 

stars  become  more  complicated,  being  either  six-rayed  or  more 
usually  nine-iayed  in  the  adult  condition  (Fig.  370). 

As  early  as  the  second  month  of  development  the  lens-  vesicle 
becomes  completely  invested  by  the  mesodermal  tissue  in  which 
blood-vessels  are  developed  i.i  considerable  numbers,  whence  the 


FlO.  370. — P083XRK«  (InnSK)  SctP ACE  OP  THE  Lens  FBOH  AN  AdULT  SHOWING  THE 
SOTUEAL  LlKZS.— (XoM.) 


investment  is  termed  the  tunica  vasculosa  lentis  (Fig.  278,  tv).  The 
arteries  of  the  tunic  are  in  connection  principally  with  the  hsraloid 
artery  of  the  vitreous  humor  (Fig.  a76),  and  consist  of  numerovi- 
fine  branches  which  envriop  the  lens  and  terminate  in  loops  almo  1 
at  the  center  of  its  ooter  surface.    This  taaic  undergoes  degenera- 


THE   OPTIC  CUP 


453 


W.  ca„.„,  .He  ™.«o™a.o„  W„  as  l^I^lt:!^  ^^Z; 

nontuS'll'''  ""^"^  ""■='  «•>'  '-''  -  '""ounded  by  a 
non-cellular  membrane  termed  the  caistUe     The  «««•«     t  L- 

assumed  by  the  optic  bulb  after  the  inva^ginatL  oTus  „u' er  w^ 

ne  defect  of  the  eye  known  as  coloboma;  this  may  vary  In  ite  extent 
sometimes  affecting  both  the  iris  and  the  retina'and  fomtog  w2 


454 


TBI   nU8  AND   CIUAIY   BODY 


Is  termed  coloboma  iridis,  and  at  others  being  confined  to  the  reti- 
nal portion  of  the  cup,  in  which  case  it  is  termed  coloboma 
chorioidc. 

Up  to  a  certain  stage  the  differentiation  of  the  two  layers  which 
form  the  optic  cup  proceeds  along  similar  lines,  in  both  the  ciliary 
and  retinal  regions.  The  layer  which  represents  the  original  inter- 
nal portion  of  the  bulb  does  not  thicken  as  the  cup  increases  in  size, 
and  becomes  also  the  seat  of  a  deposition  of  dark  pigment,  whence 
it  may  be  termed  the  pigment  layer  of  the  cup ;  while  the  other  layer- 
that  formed  by  the  invagination  of  the  outer  portion  of  the  bulb,  and 
which  may  be  termed  the  retinal  layer — remains  much  thicker  (Fig. 
a68)  and  in  its  proximal  portions  even  increases  in  thickness. 
Later,  however,  the  development  of  the  ciliary  and  retinal  portions 
of  the  retinal  layers  differs,  and  it  will  be  convenient  to  consider 
first  the  history  of  the  ciliary  portion. 

The  Development  of  (he  Iris  ahd  Ciliary  Sody.— The  first  change 
noticeable  in  the  ciliary  portion  of  the  retinal  layer  is  its  thinning  out, 
a  process  which  continues  until  the  layer  consists,  like  the  pigment 
layer,  of  but  a  single  layer  of  cells  (Fig.  271),  the  transition  of  which 
to  the  thicker  retinal  portion  of  the  layer  is  somewhat  abrupt  and 
corresponds  to  what  is  termed  the  ora  serrata  in  adult  anatomy. 
In  embryos  of  lo.a  cm.  the  retinal  layer  throughout  its  entire  extent 
is  readily  distinguishable  from  the  pigment  layer  by  the  absence  in 
it  of  all  pigmentation,  but  in  older  forms  this  distinction  gradually 
diminishes  in  the  iris  region,  the  retinal  layer  there  acquiring  pig- 
ment and  forming  the  uvea. 

When  the  anterior  chamber  of  the  eye  is  formed  by  the  splitting 
of  the  mesoderm  which  has  grown  in  between  the  superficial  ecto- 
derm and  the  outer  surface  of  the  lens,  the  Peripheral  portions  of  its 
posterior  (inner)  wall  are  in  relation  with  the  ciliary  portion  of  the 
optic  cup  and  give  rise  to  the  stroma  of  the  ciliary  body  and  of  the 
iris  (Fig.  271),  this  latter  being  continuous  with  the  tunica  vasculosii 
lentis  so  long  as  that  structure  persists  (Fig.  278).  In  embryos 
of  about  14.5  cm.  the  ciliary  portion  of  the  cup  becomes  thrown  into 
radiating  folds  (Fig.  271),  as  if  by  a  too  rapid  growth,  and  into  the 


THK   IU8  AND   QUAKY   BODY 


4SS 


foldf  Umelle  of  me<oderm  project  from  the  ttroma.  These  folds 
occur  not  only  throughout  the  region  of  the  clliaiy  body,  but  also 
extend  Into  the  iris  region,  where,  however,  they  are  but  temporary 
structures,  disappearing  entirely  by  the  end  of  the  fifth  month.  The 
toMs  in  the  region  of  the  corpus  clliare  persist  and  produce  the 
cUtary  processes  of  the  adult  eye. 

Embedded  in  the  substance  of  the  iris  stroma  in  the  adult  are 
non-stnped  muscle-fibers,  which  constitute  the  sphincter  and  dUa- 


ISIr 


Sfh.     R, 


CC 


Wnr1!j',  ^'  ^  '""*  '^'"  '"PP°^  '^'  *«^  fib*"  were  dif- 

Id  the  II    V"''    "'".*'  ""^  "'  *•''=  ?'■«"-="'  "^y-^^  "f  'he  optic 

Z'    V. ,  !^".T  *PP""""«  '''"  '^'  P"P'""''^  border  (Fig.  L, 

5M)  while  the  dilatator  is  more  peripheral  >^    8     7'. 

r*.  Z)«..fo^^  of, He  Retina.-Thronghout  the  retinal  region 

of  the  cup  the  pigment  layer,  undergoing  the  same  changes  as  in 


•«c»oeonf  issoiUTioN  ibt  cha>t 

(ANSI  and  ISO  TEST  CHART  K„,  2) 


APPLIED    IM/OE      In, 

1653  Eos)  Main  Stmt 

??fl?""-  '^*"  '^°'^        '*6M       US* 

(716)  462  -  0300  -  Phof>. 

(716)  2M-5989-Fai, 


■m 


456 


THE   RETINA 


the  ciliary  region,  forms  the  pigment  layer  of  the  retina  (Fig.  272,  />). 
The  retinal  layer  increases  in  thickness  and  early  becomes  differen- 
tiated into  two  strata  (Fig.  268),  a  thicker  one  lying  next  the  pigment 
layer  and  containing  numerous  nuclei,  and  a  thinner  one  containing 
no  nuclei.  The  thinner  layer,  from  its  posUioiL  aod  structure, 
suggests  an  homology  with  the  marginal  velum  of  the  central  nervous 
system,  and  probably  becomes  converted  into  the  nerve-fiber  layet 


grillErilSEBCEEIHSE' /> 


^^^M°6|(^fe(^. 


00  qO  00 


Fio.  a7J.— PoiTiOH  Of  A  Txuisvixsx  Section  of  the  Retina  or  a  New-bokn 

Rabbit. 
ch,  Cborioid  coat;  g,  ganglion-cell  layer;  r,  outer  layer  of  nuclei;  p,  pigment  layer.— 

(Fakhi.) 

of  the  adult  retina,'  the  axis-cylinder  processes  of  the  ganglion  cells 
passing  into  it  on  their  way  to  the  optic  nerve.  The  thicker  layer 
similarly  suggests  a  comparison  with  the  mantle  layer  of  the  ccd 
and  brain,  and  in  embryos  of  38  mm.  it  becomes  differentiated  ir.to 
two  secondary  layers  (Fig.  272),  that  nearest  the  pigment  la>cr 
(r)  consisting  of  smaller  and  more  deeply  staining  nuclei,  probaMy 
representing  the  rod  and  cone  and  bipolar  cells  of  the  adult  retii  a, 


THE    RETINA 


457 


while  the  inner  layer,  that  nearest  ths  maiginal  velum,  has  laiger 
nuclei  and  is  presumably  composed  of  the  ganglion  cells. 

Little  is  as  yet  known  concerning  the  further  differentiation  of 
the  nervous  elements  of  the  human  retina,  but  the  history  of  some 
of  them  has  been  traced  in  the  cat,  in  which,  as  in  other  mammals, 
the  histogenetic  processes  take  place  at  a  relatively  later  period  than 
m  man.    Of  the  histogenesis  of  the  inner  layer  the  information  is 


FlO.  J73._piAO»AM  SHOWINO  THE  DEVELOPMENT  OF  IHE  RETINAt  EI.EKENTS. 

.„H  J  •  1  u-  "■""  ■'"'P"'".  and  »,  in  ihe  bipolar  stage;  c,  rod  cells  in  the  uninoUr 
Ca^  '  **"''^"  ^^:  '•  "ternal  Umitiiig  lnembrane.-(jr<i«iW,  <^la 

rather  scant,  but  it  may  be  stated  that  the  ganglion  cells  are  the 
earliest  of  all  the  elements  of  the  retina  to  become  recognizable. 
The  rod  and  cone  cells,  when  first  distinguishable,  are  unipolar  cells 
(Fig.  273,  a  and  c),  their  single  processes  extending  outward  from  the 
cell-bodies  to  the  external  limiting  membrane  which  bounds  the 
outer  surface  of  the  retinal  layer.  Even  at  an  early  stage  the  cone 
cells  (a)  are  distinguishable  from  the  rod  cells  (c)  by  their  more 


4S8 


THE  OPTIC  NESVE 


decided  reaction  to  silver  salts,  and  at  first  both  kinds  of  cells  are 
scattered  throughout  the  thickness  of  the  layer  from  which  they  arise. 
Later,  a  fine  process  grows  out  from  the  inner  end  of  each  cell,  which 
thus  assumes  a  bipolar  form  (Fig.  273, 6and  d),  and,  later  still,  the  cells 
gradually  migrate  toward  the  external  limiting  membrane,  beneath 
which  they  form  a  definite  layer  in  the  adult.  In  the  meantime 
there  appears  opposite  the  outer  end  of  each  cell  a  rounded  eminence 
projecting  from  the  outer  surface  of  the  external  limiting  membrane 
mto  the  pigment  layer.  The  eminences  over  the  cone  cells  are  larger 
than  tnose  over  the  rod  cells,  and  later,  as  both  increase  in  length, 
they  become  recognizable  by  their  shape  as  the  rods  and  cones. 

The  bipolar  ceUs  are  not  easily  distinguishable  in  the  early  stages 
of  their  differentiation  from  the  other  cells  with  which  thy  are  min- 
gled, but  it  is  believed  that  they  are  represented  by  cells  which  are 
bipolar  when  the  rod  and  cone  cells  are  stiU  in  a  unipolar  condition 
(F'g-  273.  «)•  If  this  identification  be  correct,  then  it  is  noteworthy 
that  at  first  their  outer  processes  extend  as  far  as  the  external  limiting 
membrane  and  must  later  shorten  or  fall  to  elongate  until  their 
outer  ends  lie  in  what  is  termed  the  outer  granular  layer  of  the  retina, 
where  they  stand  in  relation  to  the  inner  ends  of  the  rod  and  cone 
cell  processes.  Of  the  development  of  the  amacrine  (f,  i)  and 
honzontal  ceUs  (g)  of  the  retina  little  is  known.  From  their  position 
m  new-bom  kittens  it  seems  probable  that  the  former  are  derived 
from  ceUs  of  the  same  layer  as  the  ganglion  cells,  while  the  horizon- 
tal cells  may  belong  to  the  outer  layer. 

In  addition  to  the  various  nerve-elements  mentioned  above,  the 
retina  also  contams  neuroglial  elements  known  as  Mailer's  fibers 
(Fig-  273.  *)>  which  traverse  the  entire  thickness  of  the  retina.  The 
development  of  these  cells  has  not  yet  been  thoroughly  traced,  but 
they  resemble  closely  the  ependymal  ceUs  observable  in  early  stages 
of  the  spinal  coid. 

The  DtruOopment  of  the  Optic  Nerve.— The  observations  on  the 
development  of  the  retina  have  shown  very  clearly  that  the  great 
majority  of  the  fibers  of  the  optic  nerve  are  axis-cylinders  of  the  gang- 
lion cells  of  the  retina  and  grow  from  these  cells  along  the  optic 


THE   OPTIC  NXSVZ 


459 


stalk  towaid  the  bniin.  Their  embryonic  history  has  been  traced 
most  thoroughly  m  rat  embryos  (Robinson),  and  what  foUows  is 
based  upon  what  has  been  observed  in  that  animal. 

The  optic  stalk,  being  an  outgrowth  from  the  brain,  is  at  first 
a  hollow  structure,  its  cavity  communicating  with  that  of  the  third 
ventricle  at  one  end  and  with  that  of 
the  optic  bulb  at  the  other.     When  the 
chorioid  fissure  is  developed,  it  extends, 
as  has  already  been  described,  for  some 
distance  along  the  posterior  surface  of 
the  stalk  and  has  lying  in  it  a  portion  of 
the  hyaloid  artery.    Later,  when  the  lips 
of  the  fissure  fuse,  the  artery  becomes 
enclosed  within  the  stalk  to  form  the  ar- 
teria  centralis  retina  of  the  adult  (Fig. 
276).    By  the  formation  of  the  fissure 
the  original  cavity  of  the  distal  portion 
of  the  stalk  becomes  obliterated,  and    t 
the  same  time  the  ventral  and  posterior 
walls  of  the  stalk  are  brought  into  con- 
tinuity with  the  retin.         er  01  the  op- 
tic  cup,  and  so  opp     anity  is  given  for  the    passage  of  the 
ans-cyhnders  of  the  ganglion  cells  along  those  walls  (Fig  274) 
At  an  early  stage  a  section  of  the  proximal  portion  of  the  optic 
stalk  (Fig.  37s,  A)  shows  the  central  cavity  surrounded  by  a  num- 
ber of  nuclei  representing  the  mantle  layer,  and   surrounding 
these  a  non-nucleated  layer,  resembling  the  marginal  velum  and 
contmuous  distally  with  the  similar  layer  of  the  retina.     When  the 
ganghon  cells  of  the  latter  begin  to  send  out  their'axis-cylinder 
processes,  these  pass  into  the  retinal  marginal  velum  and  converge 
m  this  layer  toward  the  bottom  of  the  chorioidal  fissure,  so  reaching 
the  ventral  wall  of  the  optic  stalk,  in  the  velum  of  which  they  may 
be  distinguished  in  rat  embryos  of  4  mm.,  and  stiU  more  clearly  in 
those  of  9  mm.  (Fig.  275,  A).    Later,  as  the  fibers  become  more 
numerous,  they  graduaUy  invade  the  lateral  and  finally  the  dorsal 


FlO.       374. — DUGKAiaUTIC 

NomjDDiAi  Skction  of  im 
J  TIC  Cup  AND  Stalk  passing 
.moDOH  THE  Chouoid  Fis- 

SUXE. 

Ah,  Hyaloid  artery;  L,  lens; 
On,  fibers  of  the  optic  nerve;  Os 
optic  stalk;  PI,  pigment  layer] 
and  *,  retinal  layer  of  the  retina. 


t6o 


THE    OPTIC  NESVE 


walls  of  the  stalk,  and,  at  the  same  time  the  mantle  cells  of  the  stalk 
become  more  scattered  and  assume  the  form  of  connective-tissue 
(neuroglia)  cells,  while  the  original  cavity  of  the  stalk  is  gradually 
obliterated  (Fig.  275,  B).  Finally,  the  stalk  becomes  a  sohd  mass 
of  nen/e-fibcrs,  among  which  the  altered  mantle  cells  ar«  scattered. 

r,ilfl^^?i,''''*'u''f  '^'"  "*'"*  '^"^  "  *"'  be  seen  that  the  sensory 
ceUs  of  the  eye  belong  to  a  somewhat  different  category  from  those  oS 
o^e  sense^rgans.  Embiyologically  they  are  a  specializedl^ortion  o  Se 
mantle  layer  of  the  medullary  canal,  whereas  in  the  other  or«ns  ftey  are 
^npheral  structures  either  represenUng  or  being  assodated  with  repre- 
^d twL  in^rf""".™" '  ?'««':°"/'''k-  Viewfd  from  this  standpXt, 
«t!nf ^f*  A  =°"'"'"«f'°n  the  fact  that  the  sensory  portion  ofTe 
retma  is  formed  from  the  invaginated  part  of  the  optic  biUb,  some  ligh! 


Fio.  37s.— Teansvime  Sections  iraouoH  thi:  Pkoxhial  Part  of  the  Optic  Stai  k 
or  Rat  Emevo,  or  {A)  9  »..  and  (B)  iTiiC-ijM^.)  ^""•'^ 

is  thrown  upon  the  inverted  arrangement  of  the  retinal  elements,  the  rod. 
amd  cones  being  directed  away  from  die  source  of  light  The  normal 
r„H  »K  1  „  i."*""''  ^^"  *"''  marginal  velum  are  retained  in  the  retina 
t,„,u  I  '^''^f^o^IPg  as  a  conducting  layer  for  the  axis-cylinders  of  th. 
Tt^^n  T/^*^*^""^  ff'.^^  'Tl"^  nerve-fibers  becomes  interposed 
between   die  source  of  light  and  the  sensory  cells.    Furthermore    i 

r^L^.T^"^  ,°"'  ^^}  "  "■*  differentiation  of  the  retina  be  im 
agined  to  take  place  before  die   closure  of  die   medullary  canal-, 

Z-tH^r  *  ■?  '"*"*'"!  '"  '"'"^  °f  ^'^  """^  vertebrites-dier, 
r.^„  •  °?  '°^"5ion  of  die  elements,  diis  peculiarity  being  due  t. 
ll  f?rrf."i.°^  '^•^  medullary  plate  into  a  tube,  and  more  espJdaUy  t, 
die  fact  diat  tiie  retina  develops  from  die  outer  wall  of  the  optidcup     I 


IHE    VITREOUS  HUMOK 


4<3l 


is  formed  from  the ?„„«  13^^  of  tS^  bulL  !"„■  li^'.h""  "'  "V^  P'"'"' ''* 
inversion  of  the  elements    ^  "'  ""=  ""'"'  »""'  '"  ""s  case  there    is   no 

<.hi^hS«telie?  :S°:erlfh  '"^"'^  "k"™  '™'"  "^^  "-^^ry 
the  retina  be  regarded  as  a  ^^t^"'^■         •""•"'  P"^"'^'''    '•""^  " 

that  the  nerve  if  not  a  ne^rjt  all  in  h.  ,7"  ."'  ""'T.'y''""'  " '»  ^''^ 
tract,  confined  througS^t  i^en  rre  t  "t  wXinl''''''  Tl' """''' 
system  and  comparable  to  such  ^ronn!    f  r  i,  V*"^  """■^'  ""™"' 

or  fillet  tracts  of  tharsystem        ^      '^         '*""  *'  *'  '"™<='  '^"»''<'>1« 

TAe  Development  of  the  Vitreous  Humnr     Tt  i,„      i      j    , 
pointed  out  (p.  4,8)  th'at  a  Uoo^ZSTC^Jr^Vrj: 
pan.ed  by  some  mesodermal  tissue  make,  its' way  into7he  caTy 


r 

••IC.  276.— RlCONSUtlTcnON  OF  A  PoBTmM  m  -m,  t:' 

Of  the  optic  cup  through  the  chorioid  fissure.  On  the  closure  nf  ,h. 
fissure  the  artery  becomes  enclosed  within  the  opdc  sta^  Ind^ 
.0  penetrate  the  retina,  upon  the  surface  TwScti^sbrnchT 
ranufy.  I„  the  embryo  the  artery  does  not,  Lweve^  teS 
in  these  branches  as  it  does  in  the  adult,  but  is  continued  nnfT  V 
t^cavit,  of  the  optic  cup  (Fig.  .;5)  to  .ach  theC^Tn S 
11  sends  branches  to  form  the  tunica  vasculosa  lentis 

According  to  some  authors,  the  formation  of  the  vitreous  humor 
IS  closely  associated  with  the  development  of  this  artery  "he  humo 
b«„g  merely  a  transudate  from  it,  while  others  havTma  mated 
that  us  a  d^vativeof  the  mesoderm  which  accompanies  th  t  "el 
and  IS  therefore  to  be  regarded  as  a  peculiar  gdatinous  fom  of 


463 


THZ    VITKI0U8  BDHO> 


connective  tissue.  More  recently,  however,  renewed  observations 
by  several  authors  have  resulted  in  the  deposition  of  the  mesoderm 
from  the  chief  rOle  in  the  formation  of  the  vitreous  and  the  substitu- 
Uon  w  It  of  the  retina.  At  an  early  stage  of  development  delicate 
protoplasmic  processes  may  be  seen  projecting  from  the  surface  of 
the  retinal  layer  into  the  cavity  of  the  opUc  cup,  these  processes 
probably  ansmg  from  those  cells  which  will  later  form  the  Mailer's 


F:o.  J77.-T.ANsv.ME  Sktion  ihmuoh  the  Causy  Rzoion  of  a  CmcK  Eio.vo 

(neurogUa)  fibers  of  the  retina.  As  development  proceeds  they  in- 
crease m  length,  forming  a  dense  and  very  fine  fibrillar  reticulum 
traversmg  the  space  between  the  lens  and  the  retina  and  constituting 
the  pnmary  vitreous  humor.  The  formation  of  the  fibers  is  espe 
cially  active  in  the  dliaiy  portion  of  the  retina  and  it  is  probable  that 
It  IS  from  some  of  the  fibers  developing  in  this  region  that  the  sus- 
pensory ligament  of  the  lens  i^cmOa  Zinmi)  (Fig.  j„,  st)  is  formed 


THE    CORNEA 


463 


generally  adSd  thaH  L^  strenuously  denied.  But  it  is 
trates  the  vitr^rto  form  .£  t  '  '''"°  ""^  ''^^°"'  ""'^'y  P««- 
it  certain  «« Je^  e  llm  ""'^  T"'"'"  '"""'  "  '='"^-  -"> 
It  has  been  ^^1^^'^^::,^^'''';  »  =".  P^^'  —tain, 
vitreous,  which,  accordta^to  flT  Wei  ro72er'''' '-^'f '^^ 
partly  ectodermal  and  p^y  mesj  rma  r^L  P.1  °T'  '^'"« 
contrary,  it  has  been  1,"  .  '"**™^™*'  (Van  P^e),  and,  on  the 

comp^e  Lenera.  on  7    T       "u'  ""^^  ^^''"'"^''^  ""dergo 
ori/n  (von  SSr  "  ""'  °'  """'^  ""^'™*' 

artery.    TWs  berir^^l  I     degeneration  of  the  hyaloid 

competed  Lnfthelth  "^^7"  'f  "'  "'^'^  '"°"''-  -<»  ^ 
exUtence  of  the  vessel S.?'  ^'-^'^  """"  ""^^  '''"h  »'  'he 
the  vitreous  hun^r^L^ror:  Zd  L"h  '""'^''"'^  "^  ""^  ''"^  "^ 
originally  occupied  brtheTter^anrr"" ''''T"''"«  '''^  ^P^« 

capsule.    Over  L  ITT  "'"""'^  ''  °='^""-  ^"""ing  a 

diLentiaUon  of  thislt;,r°"""  °'  ''l^  "P'^^  ^P  "'^  ^'er 

and  fibrous  the  foTmer  w'  V  ''^'"'"  ^""^  ""  °"'«'-  denser 

and  the  lattl  theSr       """"'^  ''^  ^^"'^'"^  ^'""  °^  ">^  ^""'t  eye 

AfteM^lSiaTJ'f''^  r^^*^^  "^  ■""-  --P'-ed. 

o;  --erm^irnitt'^^r.^-rtr^^^^^^  •- 

P'ace  to  a  layer  of  homogeneous  substance  ilwWch  J^  Z^ 


464 


THE  AMTEtlOB  CRAMBEK  Of  THE  EVE 


more  numerous  laterally  than  at  the  center,  are  embedded.  Still 
later  cells  from  the  adjacent  mesenchyme  grow  into  the  layer,  which 
increases  considerably  in  thickness,  and  blood-vessels  also  grow  into 
that  portion  of  it  which  is  in  contact  with  the  outer  surface  of  the 
lens.  At  this  stage  the  interval  between  the  surface  ectoderm  and 
the  lens  is  occupied  by  a  solid  mass  of  mesodermal  tissue  (Fig.  278, 
CO  and  tv),  but  as  development  proceeds,  small  spaces  (ac)  filled 

ac 


Fio.  378.— TuNsvnn  SicnoH  thkouoh  th»  Ciuahy  Riciom  o»  a  Pio  Eiono  ot 

33  UK. 

ac.  Anterior  chuibcr  of  the  ejre;  ce,  cornea;  tc,  ectoderm;  <,  lens;  me,  dluiT  mmcle; 
f,  pigment  layer  of  the  optic  cup;  r,  retinal  Uyer;  Iv,  tunica  rasculou  \eati>.—{Antducd.) 

with  fliiid  begin  to  appear  toward  the  inner  pordon  of  the  mass,  and 
these,  increasing  in  number  and  size,  eventually  fuse  together  to 
form  a  single  cavity  which  divides  the  mass  into  an  inner  and  an 
outer  portion.  The  cavity  is  the  anterior  chamber  of  the  eye,  and  it 
has  served  to  separate  the  cornea  (co)  from  the  tunica  vasculosa 
lentis  (tv),  and,  extending  laterally  in  all  directions,  it  also  separates 
from  the  cornea  the  mesenchyme  which  rests  upon  the  marginal 
portion  of  the  optic  cup  and  constitutes  the  stroma  of  the  iris.  Cells 
arrange  themselves  on  the  corneal  surface  of  the  cavity  to  form  a 


IHK   EVZUDS 


465 


conttouou,  endolhehal  layer,  and  the  me,,„chyme  which  forms  the 
f^l'r^  boundary  of  .he  cavity  awumcs  a  iibrou5  character  and 
forma  the  hgamenium  ^   inalum  iridis,  among  the  fibers  of  which 

B«o„'2'.J°""'°  "  f  '/^"  "/^'^•^  (Fig.  »77,  »/).  appear. 
Beyond  tlie  margins  of  the  cavity  the  corneal  tissue  is  dh^tly  con- 
tinuous  with  the  sclerotic,  beneath  the  manfin  of  which  is  a  distinctly 
thickened  portion  of  mesenchyme  resting  upon  the  ciliary  processes 
and  fonmng  the  stroma  of  the  ciliary  body,  as  well  as  givi^  rise  10 
the  muscle  tissue  which  constitutes  the  cilU^y  musde  (Figs.*;,  and 

"/0|  HtCjt 

The  ectoderm  which  covers  the  outer  surface  of  the  eye  does  not 
proceed  beyond  the  stage  when  it  consists  of  several  layers  of  cells, 
and  never  develops  a  stratum  comeum.  In  the  corneal  region  it 
resto  directly  upon  the  corneal  tissue,  which  is  thickened  slightiy 
upon  ite  outer  surface  to  form  the  anterior  elastic  lamina;  more  per- 
Vherally  however,  a  quantity  of  loose  mesodermal  tissue  iL 
between  the  ectoderm  and  the  outer  surface  of  the  sclerotic,  and, 
together  with  the  ectoderm,  forms  the  amju,>cliva  (Fig.  277,  ci) 

The  Develcpment  of  the  Accessory  Apparatus  of  the  £y«.-The 
eyeltds  make  their  appearance  at  an  early  stage  as  two  folds  of  skin, 
one  a  short  datance  above  and  the  other  below  the  cornea.  The 
center  of  the  folds  is  at  first  occupied  by  iniifferent  mesodermal 
bssue,  which  later  becomes  modified  to  form  the  comiective  tissue 
of  the  lids  and  the  tarsal  cartilage,  the  muscle  tissue  probably 
secondarUy  growmg  into  the  lids  as  a  result  of  the  spreading  of  the 
iTT  °T7,'^';*'''«'  «•«  "■■Wcularis  oculi  apparently  being  a 
denvative  of  that  she«!t  of  muscle  tissue. 

...J^-  *!*r'.  **"  ^""^  °f  ""=  'tird  month  the  lids  have  become 
sufficiently  li^e  ,0  meet  one  another,  whereupon  the  thickened 
epuhehum  which  has  formed  upon  their  edges  unites  and  the  lids 
fuse  together,  m  which  condition  they  remain  until  shortly  before 

atT"  J?'""'^^, '  u'  ',T  °^  ^"''°"  *'  ^y''"^^^  (Fig.  279,  h)  develop 
a  the  edges  of  the  lid.,  having  the  same  developmenLl  history  I 
ordinajry  hairs,  and  from  the  fused  epithelium  of  each  lid  there  ^ow 
upward  or  downward,  as  the  case  may  be,  into  the  mesodermic 


466 


THE  EVZLIDS 


ti»ue  solid  rods  of  ectoderm,  certain  of  which  early  give  off  numer- 
r]^?,.!!"  w '  S"^***""  *"'*  ^"""^  recognizable  as  the  tarsal 
Jn™^*^"^  '^'  ^'"^'  "^^^  °"'"'  ™'""  'he  simple  cylindrical 
I^  '"^;7'^'"  'he  «W,  »/  J/.,//  When  the  eyelids  separate, 
these  solid  ingrowths  become  hollow  by  a  breaking  down  of  their 


Fio.  J79. — SccnoN  razouGB  1 


».Eyd^;«.Whd;«.u™.gUnd;^^„u«I.b„„d..;W.upp.,M.-(5*^„„ 
central  cells,  just  as  in  the  sebaceous  and  sudoriparous  glands  of 

IV.  ■  I  r  'T""  ^"'"'^  '""«  ^'""^  modifications  of  the  forLe 
glan;ls  while  the  glands  of  Moll  are  probably  to  be  regarded  a 
specialized  sudoriparous  glands. 

A  third  fold  of  skin,  in  addition  to  the  two  which  produce  th, 
eyehds,  IS  also  developed  in  connection  with  the  eye,  forming  [l 
fhca  sem.!unar,s.  This  is  a  rudimentary  third  eyelid  represent^nK 
the  mctita.  ng  membrane  which  is  fairly  well  d^-elo^d  in  mant 
of  the  lower  mammals  and  especially  well  in  birds. 


THE    LACRKYHAl  GLAND 


467 


„„  Jv^  '^y^j'"^  »  developed  at  .bout  the  third  month  as  a 
number  of  branching  outgrowths  of  the  ectoderm  into  the  adjacent 
m^^m  along  the  outer  part  of  the  line  where  the  epitheliL  of 

^L?r.  "  '^'"""  '°""""''"'  *""  "«'  ^"^'^"i  'he  inner 
surface  of  the  upper  eyelid.    As  in  the  other  epidermal  glands  the 

low  by  the  degeneration  of  their  axial  cells. 

The  mso-lachrymal  duel  is  developed  in  connection  with  the 
groove  which,  at  an  early  stage  in  the  development  (Fig.  62),  extends 


Lacbbyiul  Ducn  m- 

ULIS  BIINO  KlUlED  IK 


The  q-did,  „  re^y  '"^  "  *«  .u«.  but  W  b«.  „p,^,«,  , 
for  the  sake  of  iieutuai.—(Alk.) 


I  Kptntc 


from  the  mner  comer  of  the  eye  to  the  olfactory  pit  and  is  bounded 
p<«tenorly  by  the  maxillary  process  of  the  first  vLeral  arch.  The 
epithelium  lying  in  the  floor  of  this  groove  thickens  toward  the  begin- 

In  "T  "''t '°  '°™  ^  ^"''  "^^'  ^^'^  ^^  i«°  «he  sub- 
^cent  mesoderm  From  its  upper  end  two  outgrowths  arise  which 
become  com,ected  with  the  ectoderm  of  the  edges  of  the  upper  and 

finT  t'  T^'=^"^^'y-  ""d  ^=P^=^"«  the  lachrymal  dZ,  and, 
finally,  the  solid  cord  and  its  outgrowths  acquire  a  lumen  and  a 

3  cJ^°  '"""""  membrane  of  the  inferior  meatus  of  the 

h;.  J*""  i"!*^T  ''"f  '°"°*'"  '^"'  "'''  '^''•"  °f  'he  eyelid  some 
Jstance  lateral  to  the  inner  angle  of  the  eye,  and  between  its  open- 
ing and  the  angle  a  number  of  tarsal  glands  develop.  The  supLr 
duct,  on  the  other  hand,  opens  at  first  close  to  the  inner  angle  and 


468 


UTERATUSE 


later  moves  laterally  until  its  opening  is  opposite  that  of  the  inferior 
duct.  During  this  change  the  portion  of  the  lower  lid  between  th^ 
opening  of  the  inferior  duct  and  the  angle  is  drawn  somewhat  up- 
ward, and,  with  its  glauds,  forms  a  small  reddish  nodule,  resting 
upon  the  plica  semilunaris  and  known  as  the  caruticula  lacrimalis 
(Fig.  280). 


LITERATURE. 

G.  Ai«xAMD««:  "tJeber  Entwicklung  und  Bau  des  Pars  inlcrior  Labyrinthi  d« 
Mheren  Siugethiere,"  Denkxhr.  kais.  misendt.  Acad.  Wiem,  ifalh.-Nalmw. 
Classes  Lxx,  1901. 

A.  AKOiiunci:  "Ueber  Entwickdung  und  Bau  da  yoidnai  Uvealtractu.  der  Vtrte- 

braten,"  Archiv/ar  mitrosk.  Anal.,  ax,  1881. 
F.  Ask:  "Ueber  die  Entwickelung  der  cAiuacuIa  lacrimalis  beim  Mensdier   nebst 
Bejieritungen  ttber  die  Entwickelung  der  TritaenrShrdien  und  der  Meibomschen 
Drttaen,"  AHolom.  Anxigar,  xxx,  1907. 

F.  Asi:  "Ueber  die  Entwicklung  der  Lidriinder,  der  Trilnenkarunkel  und  der  Nick- 

haut  beim  Menschen,  nebst  Bemerkungen  lur  Entwicklung  der  Trtaenabfahr- 
ungswege,"  Anal.  H^lt,  xxxvi,  1908. 

B.  Baoinsey:  "Zur  Entwickelung  der  GehSnrimecke,"  Arckh,  fir  miirasi.  Anal. 

xxvm,  1886.  ' 

I.  Brouan:  "Die  Entwickdungsgesduchte  der  GebSiknSchelclien  beim  Menschen  " 

Anal.  Htfle,  m,  1898. 
S.  Ramon  y  Ca;ai,:  "NouveUes  contributions  a  Ktude  histotogique  de  la  r<tine" 

Joum.  de  FAnal.  tide  la  Physiol.,  xxxn,  1896. 

G.  Qbkcioni:  "Ueber  den  gegenwHrtigen  Stand  der  Frage  hinsichtlich  der  Genese 

des  Glaakarpers,"  Arck.fllr  AugmheUk.,  l,  1904. 
A.  CONTINO:  "Ueber  Bau  and  Entwicklung  des  Lidrandes  bom  Menschen  "  Arch 
far  Opklkalmol.,  lxvi,  1908. 

A.  CoNTmo:  "Ueber  die  Entwicklung  der  Karunkd  und  der  plica  semUunaris  beim 

Menschen,"  Anh./Ur  Opklhatmol,  lxm,  1909. 
J.  Dbse:  "Die  eiste  Entwickelung  der  Riedmerven,"  Anal.  BefU,  DC,  1897. 

B.  I^««sc™.:  "Die  Entwickdung  der  TrUnenrShidien  bd  den  Siugetiere,"  Arckiv 

fir  Opklhalmol.,  ixn,  1906. 

a  FuCHs:  "  Bemerkungen  ttber  die  Herkunft  und  Entwidtdung  der  Gd.8rkn6diddien 
ba  Kamndien-Embrjronen   (nebst  Bemerkungen  ttber  die  Entwidcdung  des 
Knorpdskdetes  der  bdden  ersten  Visceralbogen),"  Archiv.  far  Anal  md  Phis 
Anal.  Ablh.,  Supplement,  i^S-  ' 

J.  Grabmo:  "BeitrJge  zur  Genese  des  Geschmacksorgans  der  Menschen,"  iforMa< 
ArheUen,  vil,  1898.  ^^ 

J.  A.  Hammae:  "Zur  allgemdnen  Morphologic  der  Schlundspalten  des  Mensdien. 
Zur  Entmckdungsgesdiidite  des  Mittdohrraumes,  des  ausseren  GehSramcM 
und  des  Paukenfdles  bdm  Menidlen," -liirt.  ^Mri,,,  XX,  r90l. 


UTE  BATUMI 


469 


J.  A.  HAMM*«:"StudienttberdieEmwicklun«disVordtid.rn,.,.,,j    •  • 

«der  On!«.c."  Arck.fi^  mikr„k.  Ana,..  Jc,TZ  ^  ""'  ""'«"  "'«™"- 

C.  Hemmjdt:  "Studien  ttber  den  Miuc.  dilatator  pudUI-  «m„,   a„    k 

Mcnschen,"  Arck«,M  And.  und  PkyM..  Amu.  AM.,  Supfle^nT 
A.  VOK  KoLtxxx.:  "Die  Enhricklung  und  Bedeutung  dU  ^^T^^/z^,*, 

V.  V0NMiHALK0vicz:"Na«enhohleund  IacobsoiisfIi«.n,»—     f 

Studie."^,«/.H«/„.xi,.898.      "  J"°'«°"«^  Org«,.    Eme  morphologi^he 

J.  L.  Paulei:  "Contribution  It  I'ttude  de  1'o.gane  de  lacobson  che.  IVmi, 
humain,"  BihUcp.  Anat.,  xvn,  1907.  JocoMon  chej  1  embiyon 

P.VANPtE:"Reclierchessurl'origineducotpivitrt,"^,c*re„i,flij    „„  ,^ 

Zoofofw,  ixiu  and  Lxv,  1898;  ixvni,  1899.  y /«•  jwjbkc*. 

A.  Robmson:  "On  the  Formation  and  Structure  of  the  OpUc  Nerve  and  It.  R,l,«„ 

to  the  Optic  Stalk,"  /«^  of  Ana,,  and  PkyM.,  xxi,  .8c«  *'^'™' 

G.  SPlciAU-CreiNCioNl:  "  Ueber  die  Entwidtlung  der  Trtaendrli«  heim  vr.-    v      ,. 

Arch.ftU  Othlhalmcl.,  lxix,  1908.  ranendrUse  beun  Menschen." 

J.  P.  ScaajntR:  "The  Gene=>i,  and  Development  of  the  N««,l«™^  p._.„  . 

Man,"  Amer.  Journ.  Ana,.,  Tan,  1911.  massages  m 

G.  L    Stueix.:  "On  the  Development  of  the  Membranous  Labyrinth  «,d  th, 

Acou«,c  and  Facial  Nerve,  in  the  Human  Embtyo,"  Anur.  J^.  Z^.. 

N.  VAN  DE«  Stmchi:  "L'histog&iM  des  parties  conatituante.  du  n.„m*„l.i.^- 
7^^.^  "  ""  ^^  "—  -  -  .-o^aTe  r^?t,r 

A.  Sinj:  "Zur  Anatomie  und  Entwickelungsgeschichte  der  hinteren   Irisschich,™ 
Tn^lZ  iL""^*  ""  *"'"'''"  '^'^""  '^'''^  de.'^tt?-' 

A.  Snu:  "Ueber  das  Entatehen  eines  fibriUlres  StUtarewehes  I.n  p-,k_ 

VerhUtnis  aur  Gl.ska.perfr.ge,"  Ana,.  hJ^^^  *"  ^°""^''  """^  ■"«» 

^  "^An:^,Z^.  r'^'T^""-'"'  "'^  =""-'-'  '-  """chUchen  Auge." 


CHAPTER  XVII. 


POST-nATAL  DEVELOPMENT. 


In  the  preceding  pages  attention  has  been  directed  principally 
to  the  changes  which  take  place  in  the  various  organs  during  the 
period  before  birth,  for,  with  a  few  exceptions,  notably  that  of  the 
liver,  the  general  form  and  histological  peculiarities  of  the  various 
organs  are  acquired  before  that  epoch.  Development  does  not, 
however,  cease  with  birth,  and  a  few  statements  regarding  the 
changes  which  take  place  in  the  interval  between  birth  and  maturity 
will  not  be  out  of  place  in  a  work  of  this  kind. 

The  conditions  which  obtain  dtiring  embryonic  life  are  so  dif- 
ferent from  those  to  which  the  body  must  later  adapt  itself,  that 
arrangements,  such  as  those  connected  with  the  placental  circula- 
tion, which  are  of  fundamental  importance  during  the  life  in  ulero, 
become  of  little  or  no  use,  while  the  relative  importance  of  others  is 
gready  diminished,  and  these  changes  react  more  or  less  profoundly 
on  all  parts  of  the  body.  Hence,  although  the  post-natal  develop- 
ment consists  chiefly  in  the  growth  of  the  structures  formed  during 
eariier  stages,  yet  the  growth  is  not  equally  rapid  in  all  parts,  and 
indeed  in  some  organs  there  may  even  be  a  relative  decrease  in  size. 
That  this  is  true  can  be  seen  from  the  annexed  figure  (Fig.  281), 
which  represents  the  body  of  a  child  and  that  of  an  adult  man  drawn 
as  of  the  same  height.  The  greater  relative  size  of  the  head  and 
upper  part  of  the  body  in  the  child  is  very  marked,  and  the  central 
point  of  the  height  of  the  child  is  situated  at  aliout  the  level  of  the 
umbilicus,  while  in  the  man  it  is  at  the  symphysis  pubis. 

That  there  is  a  distinct  change  in  the  geometric  form  of  the  body 

during  growth  is  also  well  shown  by  the  following  consideration 

(Thoma).    Taking  the  average  height  of  a  new-bom  male  as  500 

mm.,  and  that  of  a  man  of  thirty  years  of  age  as  1686  mm.,  the 

470 


POST-NATAL  DEVELOPMENT 


471 


height  of  the  body  will  have  increased  from  birth  to  adolescence 

1686 

7^-  -  3-37  fmes.    The  child  will  weigh  3.1  kilos  and  the  man 

66.1  kilos,  and  if  the  specific  gravity  of  the  body  with  the  included 
gases  be  taken  in  the  one  case  as  0.90  and  in  the  other  as  0.93,  then 
the  volume  of  the  child's  body  will  be  3.44  liters  and  that  of  the 

man's  71.08  liters,  and  the  increase  in  volume  will  be  ^'-°?  =20  66 

3-44 


Fio.  aSi.— CHttD  AND  Man  Dsawn  as  of  the  Sahe  Heioht.— (Lffliiw,  from  Uu 
"Grmik  of  the  Brain,"  Contemporary  Sciaice  Series  by  permissim  of  Charles  Scribner's 
Soms.) 


If  the  increase  in  volume  had  taken  place  without  any  alteration  in 
the  geonetric  form  of  the  body,  it  should  be  equal  to  the  cube  of  the 
increase  in  height;  this,  however,  is  3.37* =38.27,  a  number  well- 
nigh  twice  as  large  as  the  actual  increase. 

But  in  addition  to  these  changes,  which  are  largely  dependent 


47» 


roST-NATAL   DEVELOPMENT 


upon  differences  in  the  supply  of  nutrition,  chere  are  others  associ- 
ated with  alterations  in  the  general  metabjism  of  the  body.  Up 
to  adult  life  the  constructive  metabolism  or  anabolism  is  in  excess 
of  the  destructive  metabolism  or  katabolism,  but  the  amount  of  the 
excess  is  much  greater  during  the  earlier  periods  of  development 
and  gradually  diminishes  as  the  adult  condition  is  approached. 
That  this  is  true  during  intrauterine  life  is  shown  by  the  following 
figures,  c  Jipiled by  Donaldson: 


AgeioWeeki 

Weight  in  Gnms 

1 

AgeinWcdu 

Wdght  in  Gnuitt 

0  (ovum) 

0.0006 

"4 

635 

4 

— 

38 

1,330 

8 

4 

33 

1,700 

M 

so 

36 

3,340 

i6 

130 

40  (birth) 

3.>S0 

90 

»«5 

From  this  table  it  may  be  seen  that  the  embryo  of  eight  weeks 
is  six  thousand  six  hundred  and  sixty-seven  times  as  heavy  as  the 
ovum  from  which  it  started,  and  if  the  increase  of  growth  for  each 
of  the  succeeding  periods  of  four  weeks  be  represented  as  percent- 
ages, it  will  be  seen  that  the  rate  of  increase  undergoes  a  rapid 
diminution  after  the  sixteenth  week,  and  from  that  on  diminbhes 
gradually  but  less  rapidly,  the  figures  being  a."  follows: 


Periods  of  Weeks 

PeicentAge  Increase 

Periods  otWeelis 

Percentage  Increa 

S-I3 

13-16 
16-30 
30-34 

400 
500 
137 
"3 

34-38 
38-33 
33-36 
36-40 

93 
39 
3' 
45 

POST-NATAL  DEVELOFHENT  4*3 

That  the  same  is  true  in  a  general  way  of  the  growth  after  birth 
may  be  seen  from  the  following  table,  representing  the  average 
weight  of  the  body  in  English  males  at  different  years  from  birth 
up  to  twenty-three  (Roberts),  and  also  the  percentage  rate  of 
increase. 


•3 
14 
'5 

16 

«7 
18 

«9 


»3 


Yew 

Number  of  Cam 

451 

— 

3 

41 

103 
Tn7 

193 

334 
346 

830 
',435 
1.464 

'.599 
1,786 

3r«43 

»i95» 
3,'"8 
','3S 
3,496 

3.150 
1.438 
851 
738 
543 
5SJ 


Certain  interesting  peculiarities  in  post-natal  growth  become 

apparent  from  an  examination  of  this  Uble.    For  while  there  is  a 

♦From  a  compujion  with  other  aimilu  ubles  there  U  IJtUe  doubt  but  that  the 

weight  given  above  for  the  lecond  year  i»  too  high  to  be  accepted  ai  a  good  average 


474 


POST-NATAL   DEVXLOFIIKNT 


general  diminution  in  the  rate  of  growth,  yet  there  are  ma^ed 
irregularities,  the  most  noticeable  being  (i)  a  rather  marked  diminu- 
tion during  the  eleventh  and  twelfth  years,  followed  by  (a)  a  rapid 

I 


// 


Aim 
JMH 

•  a 

•  m 

•  I 

'  a 

•  t 

•  t 


'iTiTiMTiTiT.T.T.^.f.f.-y.T.-T.^  .T~ 


j^ji^nii!! 


Fio.  38a.— Ctoves  Showino  the  Annual  Inckask  in  Weight  in  (I)  Boys  anb  (II) 
Girls. 
The  faint  line  represents  the  curve  from  British  statistics,  the  dotted  tine  that  from 
American  (Bowditch),  and  the  heavy  line  the  average  of  the  two.    Before  the  sixth 
year  the  data  are  unreliable. — {^SUfhatson!) 

acceleration  which  reaches  its  maximum  at  about  the  sixteenth  year 
and  then  very  rapidly  diminish -s.    These  irregularities  may  be  more 

Consequently  the  percentage  increase  for  the  second  year  is  too  high  and  that  for  the 
third  year  too  low. 

It  may  be  mentioned  that  the  w^jhts  in  the  original  tabic  are  expressed  in  pounds 
avoirdupois  and  have  been  here  ccnvertr  into  kilograms,  and  further  the  figures  rep- 
resenting the  percentage  increase  have  been  added. 


POST-HATAl  DEVELOFHENT 


47S 


dearly  seen  from  the  charts  on  page  474,  which  represent  the  curves 
obtained  by  plotting  the  annual  increase  of  weight  in  boys  (Chart  I) 
and  girls  (Chart  II).  The  diminution  and  acceleration  of  growth 
referred  to  above  are  clearly  observable  and  it  is  interesting  to  note 
that  they  occur  at  eariier  periods  in  girls  than  in  boys,  the  diminution 
occumng  in  girls  at  the  eighth  and  nmth  years  and  the  acceleration 
reaching  its  maximum  at  the  thirteenth  year. 

Considering,  now,  merely  the  general  diminution  in  the  rate 
of  growth  which  occurs  from  birth  to  adult  life,  it  becomes  interest- 
ing to  note  to  what  extent  the  organs  which  are  more  immediately 
associated  with  the  metabolic  activities  of  the  body  undergo  a  rela- 
tive reduction  in  weight.  The  most  important  of  these  organs  is 
undoubtedly  the  liver,  but  with  it  there  must  also  be  considered  the 
thyreoid  and  thymus  glands,  and  probably  the  suprarenal  bodies 
In  all  these  organs  there  is  a  marked  diminution  in  size  as  compared 
with  the  weight  of  the  body,  as  will  be  seen  from  the  following  table 
(H.  Vierordt),  which  also  includes  data  regarding  other  organs  in 


ABSOLUTE  WEIGHT  IN  GRAMS. 
New-bohn  and  Addlt. 


LiT«» 

Thy- 
reoid 

Thy- 

mil* 

Suprarenal 
Bodies 

Spl<»> 

Heart 

Kidney 

Brain 

Spinal 
Cord 

141.7 

l.«i9.o 

4.«S 
33-8 

8.15 
36.9 

7  OS 
7  4 

la.6 
163.0 

,3.6 
300.6 

'3-3 
305-9 

381.0 
',430.9 

55 
39  'S 

PERCE^fTAGE  WEIGHT  OF  ENTIRE  BODy 

NZW-BOU)  AND  AdUIT. 


UVCT 

Thy- 
reoid 

Thy- 
mus 

Suprarenal 
Bodies 

Spleen      Heart 

Kidney      Brain       ^^ 

4.57 

••57 

0.16 
0.05 

o.ad 

0.04 

o.a3 

O.OI 

0.34 
0.35 

0.76 
0.46 

0.75 
0.46 

H.J9 

3.16 

0.18 

0.06 

476 


rOBT-NATAL  DEVELOPMCNT 


which  a  marked  relative  diminution,  not  in  all  cases  readily  explain- 
able, occurs. 

Recent  observatioiu  by  Hanunar  render  necessanr  some  modifica- 
tion of  the  figures  given  for  the  thymus  in  the  above  table.  He  finds  die 
average  weight  of  ^e  gland  at  birth  to  be  13.36  grms.,  and  that  the  weight 
increases  up  to  puberty,  averaging  37.5a  grms.  between  the  ages  of  11  and 
15.  After  that  period  it  gradually  diniinishes,  falling  to  16.37  grms. 
between  36  and  4s,  and  to  6.0  grms.  between  %  and  75.  Expressed  in 
percentage  of  the  body  weight  this  gives  a  value  in  the  new-bom  of  0.43 
and  in  an  individual  of  50  years  of  0.03,  a  difference  much  more  striking 
than  that  shown  in  Vierordt's  table. 

It  must  be  mentioned,  however,  that  the  gland  is  subject  to  much 
individual  variation,  being  largely  influenced  by  nutritive  conditions. 

The  remaining  organs,  not  included  in  the  tables  given  above, 
when  compared  with  the  weight  of  the  body,  either  show  an  increase 
or  remain  practically  the  same. 

ABSOLUTE  WEIGHT  IN  GRAMS. 

NSW-BOIN  AMD  AODLT. 


Skin  and  Sub- 
cutaneous Tissues 

! 

Skeleton       Musculature 

Stomadiand 
Intestines 

Pancnas 

Lungs 

6" -75 
11,765.0 

4>S-5      .         776-5 
",575.0     :     >8,73a.o 

65 
1J64 

3-5 
97.6 

54-1 
994-9 

PERCENTAGE  OF  BODY-WEIGHT. 

NlW-BO«M  AND  ADtn,T. 


Skin  and  Sub- 
cutaneous Tissues 

Skeleton 

Musculatuie 

Stomach  and 
Intestinca 

Pancreas      Lungs 

19-73 
17-77 

'3-7 
17.48 

25-05 
43-40 

3.1 
1.06 

O.Zl 

0.15 

I-7S 
1.50 

From  this  table  it  will  be  seen  that  the  greatest  increment  of 
weight  is  tliat  furnished  by  the  muscles,  the  percentage  weight  of 
which  is  one  and  three-fourths  times  as  great  in  the  adult  as  in  the 


POST-NATAL  DBVCLOPiaNT  477 

new-bom  child  as  m  the  adult,  and  the  increase  is  due  merely  to  an 
eal«gement  of  organs  already  present.    The  percentage  weight 

IJr  *^r  •""  ^'  P"""^'  ""*  '"^^  "-"^  pracUcally  the 
same,  while  in  the  case  of  the  skeleton  there  is  an  appreciable  in- 
crease, and  in  that  of  the  skin  and  subcutaneous  tissue  a  slight 


diminution.  The  latter  is  readily  understood  when  it  is  remembered 
«ut  the  area  of  the  skin,  granting  that  the  geometric  form  of  the 
body  remains  the  same,  would  increase  as  the  square  of  the  length, 
wble  the  mass  of  the  body  would  increase  as  the  cube,  and  hence 
in  companng  weights  the  skin  might  be  expected  to  show  a  diminu- 
tion even  greater  than  that  shown  in  the  table. 


478  POST-NATAL  DEVKUFMCMT 

The  increase  in  the  weight  of  the  skeleton  is  due  to  a  certain 
extent  to  growth,  but  chiefly  to  a  completion  of  the  ossification  of 
the  cartilage  largely  present  at  birth.  A  comparison  of  the  weights 
of  this  system  of  organs  does  not,  therefore,  give  evidence  of  the 
many  changes  of  form  which  may  be  perceived  in  it  during  the  pe- 
riod under  consideration,  and  attention  may  be  drawn  to  some  of 
the  more  important  of  these  changes. 

In  the  spinal  column  one  of  the  most  noticeable  peculiarities 
observable  in  the  new-bom  child  is  the  absence  of  the  curves  so 
characteristic  of  the  adult.  These  curves  are  due  partly  to  the  weight 
of  the  body,  transmitted  throagh  the  spinal  column  to  the  hip- 
joint  in  the  erect  position,  and  partly  to  the  action  of  the  muscles, 
and  it  is  not  until  the  erect  position  b  habitually  assumed  and  the 
musculature  gains  in  development  that  the  curvatures  become  pro- 
nounced. Even  the  curve  of  the  sacrum,  so  marked  in  the  adult, 
is  but  slight  in  the  new-bom  child,  as  may  be  seen  from  Fig.  283, 
in  which  the  ventral  surfaces  of  the  first  and  second  sacral  verte 
brae  look  more  ventrally  than  posteriorly,  so  that  there  is  no  distinct 
promontory. 

But,  in  addition  to  the  appearance  of  the  curvatures,  other 
changes  also  occur  after  birth,  the  entire  column  becoming  much 
more  slender  and  the  proportions  of  the  lumbar  and  sacral  vertebne 
becoming  quite  different,  as  may  be  seen  from  the  following  table 
(Aeby): 

LENGTHS  OF  THE  VERTEBRAL  REGIONS  EXPRESSED  AS  PERCENT- 
AGES OF  THE  ENTIRE  COLUMN. 


Age        . 

Cervical 

Thoracic 

Lumlnr 

New-born  duld 

ao.3 
19.7 

47-5 
46,7 
4S-6 
47  ■» 
46.6 

>6.8 

30.0 

34  2 

33' 

Male  adult           

31  ■« 

KWr-NATAt  DEVEIOPIONT  ^-g 

S^   ^  Lu  "="^°''W  approximately  the  same.    It  may  be 

So^'is^  r?'-""  '"^  '*''""""  '-'--°  "«*  'WO  vZhk 
r^tons  .»  greater  dunng  youth  t.an  in  the  adult,  a  condition  do, 

rowLrr^nlei;^  "^r  """'  -'''  '•-Vprntrof'Te 

S  o^'r^^t"**"""  "•'  P*"°^  und^consideratL. 
haJp.   K      /         '^°'"  '''*°  ""'"'  f°^  Whereas  in  the  adult  it  is 

Delow.    The  difference  depends  upon  slight  differences  in  the  form 

tt'n'ir.rrdl"'"'^  "'  ""'  "•°^"  ""^-'-^  --  '"-'^  -Pw-d 

r»,^  'T*  ""^  ''::'"'  "•'  P"^^  °'  P°wth  are  very  compli- 
cated. Cramum  and  brain  react  on  one  another,  and  hcnc7  n 
Wmony  with  the  relatively  enormous  size  of  the  br^n  a    toh 

of  the  Si  nfT.  t  „'^'  ""'  """^^  ^°°^  '''"•  »  considerable  part 
o   the  srfes  o   the  skull  are  formed  of  membrane  bones  which  at 

iT^nTT, '"  ""^'  """"^  '^">  ""-^  -"'"er  throughouTgiVe 
opportumty  for  considerable  modifications,  and,  furthermore  the 
base  of  the  skull  at  the  early  stage  still  contains  TcoZer'able 
amount  of  unossified  cartilage.  Without  entering  into  Zute  di 
t^U  .t  may  be  stated  that  the  principal  general  fhangesTwh  ,h" 
skull  undergoes  in  its  post-natal  development  are  (,)   a  rett  ^ 

^m'of  a  f  ."^^'^-'  -  >-8th.  The  portion  of  the  ^  n 
front  of  a  sanulLr  Ime  in  the  adult  skull  is  very  much  greater  than 
that  wbch  Ues  behind,  the  proportion  betweeTthe  two'^^  b^^^ 


48o 


ron-MAtAL  mvxLonmiT 


5-1,  •stimt  3:3  In  the  child  (Fnwiep).  There  hw,  therefore,  been 
•  decidedly  more  rapid  growth  of  the  Mterior  portion  of  the  tkuU, 
a  growth  which  it  ausociated  with  a  corresponding  increaK  in  the 
dorw-ventral  dimenMont  of  the  maxilUe.  TheM  bone»,  indeed, 
play  a  very  important  part  in  determining  the  proportion*  of  the 
tkuH  at  different  periods.  They  are  so  intimately  awodated  with 
the  cranial  portions  of  the  skull  that  their  increase  necessitates  a 


Fio.  J84.-SIUU.  01  A  Niw-Bom  Qnu)  Aim  of  ah  Abok  Ma»,  Dsawh  ai  <» 
A»p«oxniATK.Y  ID  Sake  She.— (H«ii*f.) 

corresponding  increase  in  the  anterior  part  of  the  cranium,  and 
their  increase  in  tins  direction  stands  in  relation  to  the  development 
of  the  teeth,  the  eight  teeth  wMch  are  developed  in  each  marilla 
(including  the  premaxilla)  in  the  adult  requiring  a  longer  bor  than 
do  the  five  teeth  of  the  primary  dentition,  these  agwn  requiring  a 
greater  length  when  completely  developed  than  they  do  in  thdr 
immature  condition  in  the  new-bom  child. 

But  far  more  striking  than  the  difference  just  described  U  that 
in  the  relative  height  of  the  cranial  and  fadal  regions  (Fig.  284). 
It  has  been  estimated  that  the  volumes  of  the  two  portions  have  a 
ratio  of  8: 1  in  the  new-bom  child.  4:  i  at  five  years  of  age,  and  a :  i 
in  the  adult  skull  (Froriep),  and  these  differences  are  due  principally 
to  changes  in  the  vertical  dimensions  of  the  maxilte.  As  mth  the 
increase  in  length,  the  increase  now  under  consideration  is,  to  a 


POtT-HATAL  DEVIlOWaKT 


481 


ceruin  ertent  »t  Imt,  uwdtted  with  the  development  of  the  teeth, 
theM  .tructure.  calling  into  existence  the  alveolar  proce«es  which 
iTprScally  wanting  in  the  child  at  birth.  But  a  more  .mpom. 
lactL  i.  the  development  of  the  maxillary  «nui«.  the  pracU^J'" 
l«lid  bodies  of  the  ma«ill«  becoming  transformed  into  hol'o**'"*; 
These  cavities,  together  with  the  sinuses  of  the  sphenoid  and  frontal 
bones,  which  are  also  post-natal  developments,  seem  'o  stand  m 
relation  to  the  increase  in  length  of  the  anterior  P^^'on  of  he  skull, 
«^„g  to  diminish  the  weight  of  the  portion  of  the  skull  m  front 
of  the  occipital  condyles  and  so  relieving  the  muscles  of  the  neck  of  a 
considerable  strain  to  which  they  would  otherwise  be  subjected. 

These  change,  in  the  proporUons  of  the  skull  have,  of  course, 
much  to  do  with  the  changes  in  the  general  prop^rUon,  "fthe  f  a.e. 
But  the  change,  which  take  place  in  the  mwdible  are  also  impor- 
Unt  in  this  connection,  and  are  similar  to  those  of  the  maxill.  in 
Sg  assoda.«i  with  the  development  of  the  teeth.    In  the  new^ 
bom  child  the  horizontal  ramus  is  proportionately  shotter    han  m 
the  adult,  whUe  the  vertical  ramus  is  very  short  and  joms  the 
torizontal  one  at  an  obtuse  angle.    The  developme     of  the  teeth 
of  A^  primary  dentition,  and  later  of  the  .hree  molars  necessitates 
«,  eloiatirof  the  hori7.onUl  r«nus  equivalen'  to  that  occ»m"8 
b  the  maxill.,  and,  at  the  wme  time,  the  separation  of  the  alveolar 
to  L  of  the  two  bones  requires  an  elongaUon  of  the  -rtical  ramus 
if  the  condyl.  is  to  preserve  iU  contact  with  the  mandibula^  fossa. 
anTthis.  again,  demands  a  diminution  of  the  angle  at  which  the 
rami  joi^  if  the  teeth  of  the  two  jaws  are  to  be  m  proper  apposition 
In  the  bones  of  the  appendicular  skeleton  secondary  epiphysial 
centers  play  an  important  part  in  the  ossification,  and  in  few  are 
hese  cente^  developed  prior  to  birth,  while  the  un.on  of  the  epipio^- 
ses  to  the  main  portions  of  the  bones  takes  place  only  toward  ma- 
turity    The  dates  at  which  the  various  primary  and  secondary 
centers  appear,  and  the  time  at  which  they  unite,  may  be  seen  from 
the  Mowing  table: 


3» 


r 


483 


POST-NATAL  DEVELOniENT 
UPPER  EXTREMITY. 


Bone 

Appearance  of 

Appearance  of  Secondary 

Fusion  of  V 

Piimaiy  Center 

Centers 

Centen 

CUvfcle 

6lkwtek. 

(At  sternal  end)  17th  year. 

30th  year. 

Scarula. 
Body 

aikwidl.         1 

2  acromial  15th  year. 

a  on  vertical  border  i6th  year. 

S  aoth  year. 

Concoid.... 

1st  year. 

Head  1st  year. 

i5lh  year. 

Great  tuberosity  3d  year. 

30th  year. 

Lesser  tuberosity  5  th  year. 

Humerus 

■jthwetk. 

Inner  condyle  5th  year. 
Capitellum  3d  year. 

i8th  year. 

Troclilea  loth  year. 

17th  year. 

Outer  condyle  14th  year. 

Ulna 

pk  wak. 

Olecranon  loth  year. 

i6th  year. 

Distal  epiphysis  4th  year. 

i8th  year. 

Radius 

ylk  wtek. 

Proximal  epiphysis  5th  year. 

17th  year. 

Distal  epiphysis  3d  year. 

aothyear. 

Capitatum 

1st  year. 

Hamatum 

3d  year. 

Triquetnim   . . 

3d  year. 

Lunatum 

4th  year. 

Multangulum 

Sthyear. 

majus. 

Navicular 

6th  year. 

Multangulum 

Sthyear. 

minus. 

Pisifonn 

ijth  year. 

Metacarpals. .. 

glkviak. 

3d  year. 

aothyear. 

Phalanges 

qlk-nlkwtik. 

3d-5th  years. 

I7th-i8th  years. 

The  dates  in  italics  are  before  birth. 


POST-NAtAL  DEVELOPMENT 
LOWER  EXTREMITY. 


483 


Bone. 

Appearance  of           Appearance  of  Secondary 

Fusion  of 

Primary  Center                       Centers 

j       Centers 

IBmn 

glhmei.                Crest  15th  year. 

[  . 

Anterior  inferior  spine  1 5  th  year. 
4lh  monlk.             Tuberosity  isth  year. 

1 

Ischium 

2  ad  year. 

4*  mmlh.           \  Crest  i8th  year. 

Cartilage  appears  at  411,  month,  ossification  in  ,d  ye 

PateUa 

ar. 

Head  ist  year. 

20th  year. 

Femur 

7tA  week. 

Great  trochanter  4th  year. 

19th  year. 

Lesser  trochanter  I3th-i4th  year. 

rSth  year. 

Condyle  gtt  mmlh. 

TibU ...   . 

7lhweek.         I 

Head  endo/glh  moiuk. 
Distal  end  jd  year. 

aist-asthyear. 
I  Sth  year. 

Fibula 

glkuxek.         I 

Upper  epiphysis  sth  year. 

a  I  St  year. 

Talus.. 

I 
7/*  maMh. 
6lkmoiUh. 
A  few  days 

Lower  epiphysis  2d  year. 

20th  year. 

Calcaneus 

Cuboid 

loth  year. 

1 6th  year. 

after  birth. 

Navicular 

4th  year. 

Cuneiforms.... 

1st  year. 

Metatarsals. . . 

glhuieek. 

3d  year. 

Phalanges 

qtk-i2tk  week. 

4th-8th  years. 

I7th-i8th  years. 

So  far  as  the  actual  changes  in  the  form  of  the  appendicular 
bones  are  concerned,  these  are  most  marked  in  the  case  of  the  lower 
limb.  The  ossa  innominata  alter  somewhat  in  their  proportions 
after  birth,  a  fact  which  may  conveniently  be  demonstrated  by  con- 
sidering the  changes  which  occur  in  the  proportions  of  the  pelvic 
mameters,  although  it  must  be  remembered  that  these  diameters 
are  greatly  influenced  by  the  development  of  the  sacral  curve 
Taking  the  conjugate  diameter  of  the  pelvic  brim  as  a  unit  for  com- 
parison, the  anteroposterior  (dorso-ventral)  and  transverse  diame- 
ters of  the  child  and  adult  have  the  proportions  shown  in  the  table 
on  the  opposite  page  (Fehling). 


484 


POST-NATAL   DEVELOPMENT 


It  will  be  seen  from  this  that  the  general  form  of  the  pelvis  in 
the  new-bom  child  is  that  of  a  cone,  gradually  diminishing  in  diam- 
eter from  the  brim  to  the  outlet,  a  condition  very  different  from 
what  obtains  in  the  adult.    Furthermore,  it  is  interesting  to  note 


Diameter. 


.  f  Conjugata  vera 
«  [  TransvraM 


Antero-postennr  . 

Transverse 

Aoteio-posterior. 
Transverse 


New-bom 
Female. 


x.oo 
r.l9 
1  0.96 
1. 01 
0.91 
0.83 


Adult 
Female. 


1.00 
1.399 
1. 19 
1.151 
1.05 
1. 154 


New-bom  | 
Male.     I 


1. 00 

1.30 
0.91 
0.99 
C.78 
0.84 


Adult 
Idale. 


1. 00 
1.394 
1. 18 
1. 14 
1.07 
»i53 


that  sexual  differences  in  the  form  of  the  pehis  are  clearly  distin- 
guishable at  birth;  indeed,  according  to  Fehling's  observations, 
they  become  noticeable  during  the  fourth  month  of  intrautenne 
development. 

The  upper  epiphysis  of  the  femur  is  entirely  unossified  at  birth 
and  consists  of  a  cartilaginous  mass,  much  broader  than  the  rather 
slender  shaft  and  possessing  a  deep  notch  upon  its  upper  surface 
(Fig.  285).  This  notch  marks  off  the  great  trochanter  from  the 
head  of  the  bone,  and  at  this  stage  of  development  there  is  no  neck, 
the  head  being  practically  sessile.  As  development  proceeds  the 
inner  upper  portion  of  the  shaft  grows  more  rapidly  than  the  outer 
portion,  carrying  the  head  away  from  the  great  trochanter  and  form- 
ing the  neck  of  the  bone.  The  acetabulum  is  shallower  at  birth 
than  in  the  adult  and  cannot  contain  more  than  half  the  head  of 
the  femur;  consequently  the  articular  portion  of  the  head  is  much 
less  extensive  than  in  the  adult. 


POST-NATAL  DEVELOPICENT 


485 


It  is  a  well-known  fact  that  the  new-bom  child  habitually  holds 
the  feet  with  the  soles  directed  toward  one  another,  a  position  only 
reached  in  the  adult  with  some  difficulty,  and  associated  with  this 
supination  or  inversion  there  is  a  pronounced  extension  of  the  foot 
(i. «.,  flexion  upon  the  leg  as  usually  understood;  see  p.  102),  it  being 
difficult  to  flex  the  child's  foot  beyond  a  line  at  right  angles  with  the 
axis  of  the  leg.  These  conditions  are  due  apparently  to  the  ex- 
tensor and  tibialis  muscles  being  relatively  shorter  and  the  opposing 
muscles  relatively  longer  than  in  the  adult,  and  with  the  elongation 
or  shortening,  as  the  case  may  be,  of  the  muscles  on  the  assumption 


Fio.  »8s.-LoNoiTumNAi  Sections  or  rai  Hiad  op'the  Feiiot  of  I.A)  Niw-bohn 

Child  AND  (S)  A  Latek  Stage  OF  Devxlopment. 

»A  Epiphysial  center  for  the  head;  *,  head;  I,  trochanter.— (ff«i*e.) 

of  the  erect  position,  the  bones  in  the  neighborhood  of  the  ankle- 
joint  come  into  new  relations  to  one  another,  the  result  being  a  modi- 
fication of  the  form  of  the  articular  suriaces,  especially  of  the  talus 
(astragalus).  In  the  child  the  articular  cartilage  of  the  trochlear 
surface  of  this  bone  is  continued  onward  to  a  considerable  extent 
upon  the  neck  of  the  bone,  which  comes  into  contact  with  the  tibia 
in  the  extreme  extension  possible  in  the  child.  In  the  adult,  however, 
such  extreme  extension  being  impossible,  the  cartilage  upon  the  neck 
gradually  disappears.    The  supination  in  the  chUd  brings  the  talus 


486 


UTERATCKE 


in  close  contact  with  the  inner  surface  of  the  calcaneus  and  with 
the  sustentaculum  tali;  with  the  alteration  of  posiUon  a  growth  of 
these  portions  oi  the  calcaneus  occurs,  the  sustentaculum  becom- 
ing higher  and  broader,  and  so  becoming  a  obstacle  m  the  way  of 
supination  in  the  adult.  At  the  same  time  a  Plater  ^^^-^t  of  the 
outer  surface  of  the  talus  comes  into  contact  with  the  lateral 
malleolus,  with  the  result  that  the  articular  surface  is  considerably 
increased  on  that  portion  of  the  bone.  Marked  changes  in  the  form 
of  the  talo-navicular  articulation  also  occur,  but  their  consideraUon 
would  lead  somewhat  further  than  seems  desirable. 

LITERATURE. 
C   A.Bv:  "Die  AUersv.rschi«i.nhd«n ,d«  men^hlich«.  Wirbd^ul.."  ArcHiv  /Or 

Kinder,"  JahrlmchM  Ki«d«rheilh«de,  xxxvi,  i^3- 

fOlalleben  des  M«.schen,"  Ardm,  ^  Ana,,  und  Phys..  Ana,.  AUk..  SupfUmm,. 

''"*■     ..  -J    ir-^A.^tt^"HandlmhiUrKindvkrttnkha,m(airiar(li), 

W  Hinie:"  Anatomic  desKmdeisalters,   ji  inK>ra<-»  ■«»  " 

,  ci:^— -".".  *  <.«»-  - «"  o-»  *■  — -»- 

B«tAndlhe  ie  des  menschUdien  KSrpers,"  Leipiig,  i8«2-  „ 

H.  V»«i^T  "  ;^»mi,chc,  Physiologi^he  und  Physikalisch.  Daten  und  TabeU«, 

H.  ^^cil'f  "Unlcrsuchungen  Uber   Wachsthun.   und   Bau    d«   m««hUcl.«. 
Schadels,"  Leipzig,  1862. 


INDEX 


After-birth,  137 
After-brain,  387 
Agger  nasi,  176 
Auantois,  113,  361 
Alveolo-Ungual  glands,  294 

groove,  390 
Amitotic  division,  7 
Amnion,  108,  log 
Amniotic  cavity,  54 
Amphianhrosis,  188 
Amphiaster,  4 
An^blast,  aai 
Annulus  of  Vieussens,  333 
Anterior  commissure,  405 
Anthelix,  446 
Antitragus,  446 
Anus,  383 
Aortic  arches,  343 

bulb,  331 

septum,  336 
Archenteron,  48,  380 
Archoplasm  sphere,  4 
Arcuate  fibers,  391 
Areas  of  Langerhans,  313 
Arrectores  pilorum,  147 
Arteries,  340  ' 

anterior  tibial,  353 

aorta,  344 

branchial,  343 

carotid,  343 

centralis  retine,  450 

coeliac,  346 

common  iliac,  345 

epigai;tric,  350 

external  iliac,  347,  353 
maxillary,  343 

femoral,  354 

hyalmd,  448 

hypogastric,  347,  368 

inferior  mesenteric,  346 

innominate,  344 

intercostal,  245 

internal  nuunmary,  350 
maxiUaiy,  343 
apermatic,  346 


Arteries,  interosseous,  351 

lingual,  343 

lumbar,  345 

median,  351 

middle  sacral,  345 

peroneal,  354 

popliteal  353 

posterior  titnal,  355 

pulmonary,  343 

radial,  353 

renal,  346 

saphenous,  353 

sciatic,  353 

subclavian,  345 

superficial  radial,  351 

superior  intercostal,  348 
mesenteric,  346 
vesical,  347 

temporal,  343 

ulnar,  351 

umbilical,  116,  341,  347 

vertebral,  348 

vitelline,  119,  333 
Articular  capsule,  188 
Ary-eplifjlottic  fo'ds,  335 
Arytenoid  cartilages,  336 
Aster,  4 
Atresia  of  duodenum,  306 

of  puittl,  453 
Atrial  septum,  333 
Atiio-ventricular  valves,  338 
Auerbach,  plexus  of,  430 
Auricle,  445 
Axis  cylinder,  378 


B 

Bartholin,  glands  of,  363 
Belly-stalk,  68,  114 
Bile  ca^nllaries,  309 
Bladder,  359 
Blastoderm,  43 
Blastopcfff,  4$,  54,  57 
Blastuia,  39 
Blood,  334 

islands,  333 


487 


488 

Blood  platdeo,  119 

nssels.  111 
Body  cavity,  +8 
Bone,  devdopment  of,  154 

growth  of,  157 
Bone-marrow,  156 
Bones: 

atlas,  163,  165 
am,  165 

carpal,  184, 187,  481 
clavicle,  183,  48" 
coccyx,  166 
conciHe,  «7* 
epistropheus,  163,  105 
ethmoid,  174 
femur,  186,  483,  484 
fibula,  186,  483 
frontal,  178 
humerus,  184,  483 
hyoid,  183 
iUum,  186,  483 
incus,  179,  440 
innominate,  185,  483 
interparietal,  173 
ischium,  186,  483 
lachrynial,  178 
malleus,  I79>  440 
mandible,  x8o 
maxilla,  179 
metacarpal,  185,  483 
metatarsal,  188,  483 
nasal,  178 
occipital,  170,  I7» 
palatine,  179 
parietal,  178 
patella,  186,  483 
periotic,  169, 176  „     „      „ 
phalanges,  185,  i88,  483,  483 
prema^Ua,  179 
pubis,  186,  483 
radios,  184,  483 
ribs,  162,  165 
sacrum,  165 
scapula,  183,  483 
sphenoid,  173 
supes,  441 
sternum,  166 
suprasternal,  r66 
tarsal,  187,  483 
temporal,  176 
tibia,  186,  483 
turbinated,  r7S 
ulna,  1S4,  4S2 
vertebne,  160,  104,  478 
vomer,  175 
zygomatic,  1^8 
BracUa  conjuucUva,  394 


INDEX 


Biain,  38«,  475 
Branchial  arches,  90,  97 

clefts,  90 

epithelial  bodies,  394,  »9S 

fistula,  91 
Branchiomeres,  8x 
Bronchi,  333       .  ,,, 

Bucconasal  membrane,  3B3 
Bulbo-urethral  t,-*nds,  303 
Bulbo-vestibular  glands,  3O3 
BurdTch,  fasciculus  of,  385 
Bursa  omentalis,  324 


Cecum,  301,  305 
Calcar,  40> 
Canal  d  %  463 

of  G>  -  -,  3S7 
of  Nuck,  365 
of  Petit,  463     „ 
Canalised  fibrin,  138 
'        CapUlalies,  334  . 

Cartilages  of  Santormi,  330 

of  Wrisberg,  3^6 
CaruncuUi  laciimalis,  408 
Cauda  eqiuna,  384 
Caul,  113 
CeU,  I,  3 

diviuon,  4 
theory,  i 
CentTOaome,  4 
Cerebellum,  393 
Cerebral  aqueduct,  395 
convolutions,  403 
cortex,  407 
henuspheres,  394 
peduncles,  394 
Cheek  groove,  391 
Chin  ridge,  lOO 
Chondrocramum,  169,  iT 
Chorda  canal,  57 
doisalis,  75 
endoderm,  75 
Choridd  coat,  449.  403 

plexus,  389,  397.  4°.' 
ChonoiJd  fissure  of  brain,  401 

of  eye,  448,  453 
Chorion,  67,118 
frondosum,  134 
Uevc,  134 
Chorionic  villi,  123 
Chromaffine  organs,  370 
Chromatin,  3 
Cnromosomes,  4 
accessory,  is 
reduction  of,  14,  30 


INDEX 


489 


CUUr;  body.  4%^ 

guigtioa,4>4 

mtude,465 
Ciitema  chyli,  370 
Cleft  pftUte,  3S4 

stemum,  168 
Clitoris,  363 

ClOftCA,  380,  360 

Cloacal  membrane,  387 
Cloquet,  canal  erf,  463 
Coccygeil  gangUcm,  375 
Ccelom*  48,  78 
CoUateru  emuwnce,  404 
OrfUculiu  leminalis,  357 
Coloboma,453 
Colon,  303 
Omjunctiva,  465 
Connective  tissues,  153 
Cornea,  449,  464 
Comiculate  cartilages,  336 
Corona  radiata,  31,  353 
CtHonazy  sinus,  333 
Cwpora  mamiliana,  398 

quadrigenuna,  395 
Corpus  albicans,  34 

callosiun,  405 

luteum,  33 

striatum,  400 
Corti,  si^ral  wgan  ci,  437 
Cowper,  i^ds  of,  363 
Cramal  nerves,  409 

sinuses,  355 
Cricoid  carriage,  336 
Cundfivm  cartilages,  336 
Cutu  plate,  80 
Cytf^fMsm,  3 
Cjrto-troi^blast,  X33 


Darwin's  tubercle,  446 
Deddua  basalis,  133 

capsularis,  13 1,  131 

reflexa,  131 

serotina,  133 

vera,  130 
Decidual  cdls,  131,  137 
Dendrites,  379 
Dental  ^oove,  385 

papilla,  385 

shelf,  385 
Dentate  gyrus,  403 
Dermatome,  80 
Descent  of  ovaiy,  365 

of  testis,  366 
IHaiAragm,  330 


Diarthrasis,  288 
Diencephakm,  387,  396 
Discus  proligerus,  19,  353 
Double  monsters,  46 
Duct  of  Santmini,  313 

at  Wrisberg,  31a 
Ductus  arteriosus,  344,  a68 

Botalli,  344 

choledochus,  307,  308 

cocblearis,  434 

Cuvieri,  2^7 

ejaculatonus,  355 

endoljrmphaticus,  433 

reuniens,  434 

venosus,  360 
Duodenum,  303,  303,  306 


Ear,  431 

Ebner,  glands  of,  431 
Ectoderm,  48 
Embryo,  age  of,  io3 

external  form,  86 

growth  of,  473 
Embryonic  disc,  54 
Embryotroph,  133 
Enamel  organ,  385 
Enchylema,  3 
Endocardium,  339 
Endoderm,  43 
Envelojnng  Layer,  43 
Ependymal  cells,  377 
Ef^blast,  48 

E[nbranchial  placodes,  417 
Epidermis,  141 
E[ndidymis,  354 
Epiglottis,  335 
E|nphyses,  156 
£|nphy^  cerebri,  396 
E{nploic  foramen,  334 
Epistemal  cartilages,  166 
E[ntrichium,  141 
Eponychium,  145 
Epo6phoron,  356 
Erythrocytes,  335 
Ei^thitiplastids,  336 
Eustachian  tube,  394,  440 

valve,  234 
Extrauterine  pregnancy,  33 
Eye,  446 
Eyelids,  465 


Fallopian  tubes,  357 
Fasciculus  commtmis,  414 


49* 

Fudculu  of  Burdtch,  38s 
o(GoU,38s 
soBttritu,  4<4 
Fenatn  cochlw,  440 
ovalis,  440 
rotunda,  440 
vadbuli,  440 
FertiUutian  of  ovum,  31 
Fetal  circulation,  366 
Fibrinoid,  198 
Fifth  ventricle,  406 
Filum  tenninale,  384 
Fimbria,  405 

ovarica,  357 
Foliate  papilla,  43' 
Fontana,  spaces  of,  405 
Foramen  ctecum,  396 
of  Winslow,  334 
ovale,  333,  340 
Fore-br^n,  386 
Formado  retic-laria,  390 
Fotnii,  40s 
Frontal  sinuses,  176 
Funiculus  cuneatus,  385 

gracilis,  385 
Furcula,  394 


Gartner,  canals  of,  357 
Gall  bladder,  307,  308 
Ganglionated  cord,  433 
Gastral  mesoderm,  50,  03 
Gastrula,48 
Geniculate  bodies,  398 
Genital  folds,  363 
ridge,  338,  349 
swdling,  363 
tubercle,  363 
Germ  cells,  7 
layers,  47,  60 
plasm,  8 
Giraldes,  organ  of,  354 
Glands  of  Bartholin,  363 
bulbo-urethral,  363 
bulbo-vestibular,  363 
of  Cowper,  363 
of  Ebner,  431 
Meibonuan,  466 
of  MoU,  AfA 
salivary,  393 
tarsal,  4O6 
Goll,  fasciculus  of,  385 
Graafian  follicle,  19 
Great  omentum,  334 
Groove  of  RosenmuUer,  395 


INDEX 


Gubcmaculum  testk,  356 
GynBconiaatia,  151 


Hsmatopoiedc  organs,  335 
HKmolymph  nodes,  373 
Hairs,  146 

Hare  Up,  100,  179         „ 
Hassan's  corpuscles,  39II 
Haversian  canals,  ijS 
Head  cavities,  79 

process,  56,  69 
Heart,  339,  475 
Helix,  446 
Hensen's  node,  56 
Hermaphioditism,  365 
lOnd-br^,  3^7 
Hippocampus,  403 
Hyaloid  canal,  463    . 
Hydatid  of  Morgagm,  355 
stalked,  359 

Hydramnioa,  113 

Hymen,  357 

Hypertheba,  151 

Hypertrichoais,  148 

HypoUast,  48 

Hypochordal  bar,  JOi 

Hypophyus,  399 

Hypospadias,  365 

Hypothalamic  region,  398 


Im[dantation  of  ovum,  119 

Infracardial  bursa,  345 

Infundibulum,  399 

Ingiunal  canal,  367 

Inner  cell  mass,  44 

Insula,  404 

Interarticular  cartilages,  189 

Intercarotid  gangUon,  373 

Intermediate  cell  mass,  77 

Interrenal  organs,  370 

Interventricular  foramen,  400 

Intervertebral  fibro-cartilage,  163 

Intestine,  301,  47* 

I™.  454       ,   .     „ 

Isthmus  cerebri,  387,  393 

J 

Jacobaon,  organ  of,  439 

Joints,  188 

Jugular  lymph  sac,  386 


unmx 


49X 


Kuyokineiii,  7 

Kidney  (aee  Metanephra),  343,  475 


L»bU  majon,  363 

imnoi»,363 
Ltchrymal  guud,  467 
1*1  mim  tenninalis,  399 
Lftngerhkiu,  areas  01,  313 
lAnghaiw  cells,  136 
Lanugo,  147 

Lateral  thyrecncb,  299 

Lens,  447,  450 

Lesser  omentum,  334 

Leukocytes,  337 

Ligaments: 

broad,  oi  uterus,  349,  356 
coraco-humeral,  3 16 
ovonary,  of  liver,  331 
faldfonn,  of  liver,  331 
fibular  lateral,  of  knee,  300 
flavan,  163 

«WU«**,  349.  355»  357 
tntenfnnous,  163 
of  the  ovary,  ^58 
pectinatum  iridis,  463 
iDund,  of  liver,  368 
round,  of  uterus,  358 
tacro-tuberous,  sec 
spheno-mandibular,  180 
Busfwnsory  of  lens,  463 

Limbs,  90,  100 

Up-ridge,  100 

lips,  384 

Liver,  306,  475 

LnnM,  331,  476 

Ltuchka's  ganglion,  375 

Lymphatics,  268 

Lympt  ^odes,  373 
sacs,  368,  370 

Lympbocytes,  337,  373 

M 

Mammaiy  gland,  148 
Mandibiuar  process,  93 
Mastoid  cells,  443 
Maturation  ol  ovum,  28 
Maiallaiy  antrum,  176 

isocess,  92 
Meckel's  cartU^e,  171,  179 

diverticulum,  113,  305 


Mediastina,  333 
MeduUa  oUcngata,  387 
Medullary  canal,  73,  88 

ft^ds,  70,  73 

groove,  70 

sheath,  383 
Megacaivocjrtes,  338 
MelbomUn  glands,  466 
Meissner,  plexus  of,  430 
Membrana  pupillaris,  453 

reuniens,  81 

tectoria,  437 
Membrane  bcme,  154 
Menstruation,  36 
Mesamoelx^ds,  232 
Mesencephakm,  387^  395 
Mesenchyme,  61 
Mesenteriole,  337 
Mescnteiy,  333 
Metocardjum,  316 
Mesoodon,  336 
Mesoderm,  48 

somatic,  78 

splanchnic,  78 

ventral,  77 
Mesodermic  somites,  73,  76 
Mesogastrium,  334 
Mesonephrcs,  341 
Mesorcbium,  367 
Mesothdium,  61 
Metamere,  83 
Metanephros,  343 
Metencephalon,  387,  393 
Mid-liram,  387 
Middle  ear,  440 
Milk  ridge,  148 
Mitosis,  7 
MoU,  glands  of,  466 
Montgomery's  glands,  150 
Moi^agni,  hydatid  erf,  355 
Morula,  43 
Mouth  cavity,  383 
MuUerian  duct,  347 
Muscle  plates,  80 
Muscles: 

arrectures  pilorum,  147 

biceps  femoris,  3i6 

branchiomeric,  306 

chondroglossus,  208 

ciliary,  465 

coccygeus,  204 

constricten*  ol  pharynx,  208,  299 

cranial,  305 

curvator  coccy^,  304 

depressors  (A  hyoid,  303 

digastric,  306 

dilatator  iridis,  455 


-mm 


49a 

MiMcte,  dcful,  MO 
•ye,  MS 
faddl,<o6 

gadioaicmtiii,  »S-  "9 
geniobnid,  KM 
•enlo^Mii*,  w> 
doMO-pakliaiii,  ><» 

hnwduteuO,  109 
IntcrcMtil,  101 
UiyoKOl,  J08 
IttUiUniB  rioni,  19^ 
kntor  ul,  904 
ttmb,  310 
Imgia  oipidi,  90> 

colli,  lOl 
hunbilcal,  >i8 
muHter,  »6 
mylohroM,  «o6 
obliqui  abdominis,  >o> 
ocdptto-faoottUi,  198,  ao6 
omayM,  198 
pectoral!,  916 
perineal,  904 
peroneui  longut,  9ie 
platjnma,  906 
pronator  quadimtua,  110 
paoal,  904 
pterrgoidi,  906 
pnunidaUt,  909 
mtut  abdondnia,  199,  909 
aacro-spinalis,  199,  904 
■caleni,  909 
lemd  poateriora,  199 
lorattts  anterior,  199 
akeletal,  19; 
aoleus,  915,  919 
sphincter  ani,  904 
doaoe,  90s 
iiidis,  455 
stapedius,  906,  44r 
steroohjroid,  198 
atemomasUwl,  198,  9oj,  90B 
atjrlocloaaus,  909 
styknyoid,  906 
styiopnaryngeus,  908,  999 
temporal,  906 
tensor  trmpani,  906,  440 

veU  palati,  906 
transversus  abdominis,  9oa 

thoracis,  909 
trapeuua,  198,  ioa,  908 
Muscle  tissue,  J  93 
Myelencephalon,  387,  389 
Myelin,  389 
MyekxTtes,  997 
Myoblasts,  19s 


noBX 


MyecaidluiB,  tt9 
li^alom*,r 


«o.r9» 


N 

Nalk,i44 
Nap*  bend,  90 
Naaalpit,99 

pn>c«aa,9g 
Nato-lachrymal  duct,  467 
Nephrogenic  cord,  34> 
Nepbroatrane,  340 
Nephrolome,  w> 
Nem  componenta,  4<o,  413 

roots,  380 
Nerrea: 

aoditccy,  41 S 
cranial,  409 
h)rpo|^oMal,4«9 
oUa<:tary,498 
opdc,  458 
recurrent,  337 
'  apioal,  408 

accessory,  410 
sptancbnic,  494 
Nerre  tisaue,  377 
Neural  crest,  380 
Neurenteric  canal,  58,  69,  73 
NeoroUaats,  378  . 
Neuroglia  cdk,  |;8,  379 
Neuromeres,  418 
Neurone  theoiT,  389 
Nitabuch'a  stria,  135^      „ 
NoD-sezual  repnductiao,  a 
Nonnoblasts,  196 
Nolochoed,  74     ^ 
Nuck,  canal  d,  305 
Nucl«iB,4 
Nucleus,  3 


CEsophagus,  999 
(btrus,  97 
Odontoblasts,  987 
Olfactory  lobes,  40« 

organ,  498 
Olivary  body,  390 
Omentum,  394 
OiScyte,  9Q 
Optic  cup,  448,  453 

recess,  399 
Oral  iossa,  88,  99,  980 
Organ  of  Giraldes,  354 

of  Tacobaoo,  499 

of  RosenmUiler,  356 


nanx 


493 


Orpot,  B 

of  tMta,  430 

of  Zuckerkandl,  374 
Otteobluti,  154 

Otoc]rit,43» 

Otic  guglloB,  424 

Ot«rr,  35a 

dcKent  Of,  365 

Ovulatkm,  11,  a6 

Ovum,  19 

ferdUsatioa  of,  31 
impUi.tatioD  of.  119 
nutuntion  of,  aS 
■egmentAtion  of,  38 


PaUte,  983 
Fancreo,  311,  476 
Pvididynni,  354 
Fumphysii,  397 
Panthymut,  299 
Fuathmoid  bodies,  397 
FuoSidioroa,  356 
Farood  gland,  39a 
Parovaxuun,  356 


Ihnli,  364 
Pericardial  cavity,  317,  318 
Perineal  body,  36a 
Pwiooyx,  145 
Periosteum,  155 
Periodc  capsule,  169,  176 
Peritoneum,  333 
Petit,  canal  of,  463 
PflUfer's  conk,  353 
Flia^mgeal  buna,  394 

membrane,  aSo 

tondl,  394 
Pharynx,  394 

Phaiyngo-paiatine  arches,  283 
Pii»«l  oocly,  396 
Pinna,  445 
Ktuitary  Sody,  399 
Placenta,  133,  137 

accewory,  ia6 

embryotrophic,  133 

hcmatrophic,  133 

praevia,  133 
Placentar  infarcts,  135 
Plaamodi-trophoblast,  isa 
Plasmodium,  133 
Pleune,  333 

neuro-THnitoneal  cavi^,  78,  330 
nica : '  .lilunaiis,  466 


Polar  globult*,  30 
Polycaryocytcs,  a  38 
Polymutia,  151 

EoO«P«nny»34 
Pons,  39a 

flonire,  389 
Post-anal  fut,  381 
Pott-natal  devdopment,  470 
Posterior  lymph  sac,  370 
Procaudal  recess,  aSi 
Precoracoid,  189 
Prepuce,  364 
Primitive  groove,  56,  69 

streak,  50,  69 
Processus  globularis,  99 
Pronephric  duct,  339 
Pronephros,  339 
Proouclei,  31 
ProoestrutD,  a? 
Prostate  i^d,  363 
Proetcnnial  mesoderm,  50,  58 
Protoplasm,  a 
Pzotovcrtebne,  77 


R 

Rathke's  pouch,  385,  399 

Rauber's  covering  layer,  44 

Rectum,  a8i 

Red  nucleus,  305 

Reducti(m  of  chromosomes,  14,  30 

Restiform  body,  391 

ReteCOTds,  349 

ovarii,  354 

testis,  35a 
Retina,  455 

Retroperitoneal  lymph  sac,  370 
Rhinoicnilialon,  407 
RoscnmiiUer,  groove  of,  395 

orguk  of,  356 


Sacculus,  434 
Sacral  bend,  90 
Salivary  glands,  a  91 
Santonni,  cartilages  of,  336 

duct  ctf,  313 
Sarcode,  1 
Scala  tympani,  440 

veatibuli,  439 
Scterotic  coat,  449,  463 
Sclerotome,  80 
Scrotum,  364 


iH-PHWI 


494 

SdbKmM  fludi,  <47 
StgnHattlka  of  oran,  jS 
SmkiiciiUr  duett,  413 
SonUimu  nhm,  >J9 
SeniBUaaitt  tubvlo,  if 
8(ptam  peUaciduia,  4» 
pitmiun,  ij] 
lecuadum,  tu 
tpiuium,  13* 

tnuannuni,  318,  310,  313 
Sertoacell,  14 
Sex  cella,  349 

cordt,  349 
Sexual  reproductloil,  8 
Sinusoid,  113 
Sinui,  corourjr,  't' 
poculmiK  "i-S 
pmcervli  aii^  97 
tennin.tlis,  2i» 
venos'js,  230 
Sinn  invenui  viKtium,  46 
Skin,  141,  47' 
SItuU,  iM,  479 
Soda  paiolidi^,  191 
Solitarv  faacicului,  390 
Somatic  ceili,  7 
Spaca  of  Fontana,  463 
Spermadc  cord,  367 
Spermatid,  14 
Spermatocyte,  14 
Spermatapnesii,  13 
Speimatogania,  14 
SpermatoioOn,  it 
Sphenoidal  cdb,  176 
Spheno-paladne  ganglion,  414 
Spinal  cord,  383, 475 

nerves,  408 
Spiral  organ  o<  Corti,  437 
Spleen,  174,  473 
Stomach,  301 
Sublingual  ganglion,  4>4 

gland,  193 
Submudllary  ganglion,  434 

gland,  991 
Substance  islands,  sia 
Sudoriparous  glands,  148 
Sulcus  Monrd,  396 
Superfefadon,  36 
Suprabranchial  placodes,  417 
Suprarenal  bodies,  370,  475 

accessMy,  373 
Supratonsillar  fossa,  195 
Suture,  188 

Sympathetic  nervous  system,  418 
Synaiondr<»U,  x88 
Syncytium,  laa 
Systaiis,a 


DIDBX 


TaU  fiUmnI,  94 

Tanal  glands,  4A6 

TasM  organs  cl,  43a 

Teeth,  a8s 

Tegmentum,  394 

Tefencephaloa,  386.  39> 

Tertls,350 

descent  of,  3M 

Tfialaml,  397 

Thebaian  valve,  1^ 
Thoracic  duct,  J7« 
Thymus  gland,  197.  47* 
Thyreoid  cartilage,  33S 
gland,  196,  475       , 
ThymvgloMal  duct,  190 
Tissues,  a 
Tongue,  389 
Tonsils,  a9S 
Touch,  organs  of,  430 
Trachea,  334 
Tragus,  440 
Trophoblast,  55 
'    Tuba  audidvs,  440 
TubE  uteiiMC,  357 
Tuber  cinereum,  398 
Tuberculum  impar,  tag 
Tunica  vaginalis  testts,  307 

vascuEisa  lentis,  45' 
Tween-biain,  387 
T«in-devcl^pmeu^  4t 
Tympanic  cavity,  44s 
membrane,  443 


Ultimo-branchial  bodies,  399 
Umbilical  cord,  9a,  116 
I  .  .bilicus,  86 
Urachus,  115,  3*' 
Ureter,  344 
Urethra,  361 
Urogenital  anus,  300 
Uterovaginal  canal,  349 
Uterus,  357,  359 

mascuUnus,  355 
Utriculus,  434 

pnataticus,  355 


VaKinsL  35? 
Vaginal  process,  3O5 
ViSate  pa[dllae,  43° 


IMISX 


495 


Vm  dcfanm,  355 
Vila: 

uloior  cwdiaal,  >55 

■mndiat  lumbar,  fi4 
•mn,i«4 
bMOk.tds 
cephafic,  a6$ 
imlMan,  isq 
exttnul  jugular,  158 
h«niai)r|oi,  164 
hcpadc,  >6a 
Inieiior  vena  cava,  a6j 
Innominate,  958 
Internal  jugolar,  2t$ 
lugulo-ctphallc,  105 
omb,  36c 

long  laphenoua,  365 
portal,  a6i 

poeterior  cardinal,  355 
ptimaiy  fibular,  365 

ulnar,  365 
pulmonary,  305 
renal,  363 
•ubcardlnal,  363 
superior  vena  cava,  358 
lupncardlnal,  363 
iupfarenal,  363 
umbilical,  zi6,  3<o 
vitelline,  333,  359 
Vdum,  anterior,  394 
interpoaitum,  397 
marginal,  378 


Velum,  poaterior,  389 
Venlricular  leplum,  336 
VermUotm  appendix,  305 
Vemiz  caaeoaa,  113, 147 
Vertex  bead,  86 
Vcikula  leminalla,  335 
Vicuiaena,  annulus  of,  333 
V1IU,  chorionic,  113 
intcMinal,  305 
Vitrcoui  humor,  449,  461 
Vulva,  363 

W 

Whartofi'i  Jelly,  iig 
Winalow,  foramen  of,  334 
Winung,  duct  of ,  3 1 .1 
Wiuh  milk,  131 
WoUSan  body,  341,  354 

duct,  339,  3H 

ridge,  338 
Wrisbeig,  cartilage  of,  336 


Yolk  sac,  86,  113 
stalk,  86,  90,  113 


Zona  pellucida,  31 
Zuckerkandl,  organ  of,  374 


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Elementary  Zoology.  A  Text-book  for  Secondary  Educational 
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GREEN.  Vegetable  Physiology,  An  Introduction  to.  By  J.  Reynolds 
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VINAL.  A  Guide  for  Laboratory  and  Field  Studies  in  Botany.  By 
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